The fact of the matter (I hope everyone who doubts this will be happy to hear) is that the Catholic Church has fairly definitively settled this issue. It is NOT something "we are allowed to disagree with."
The definitive statement on the supernatural creation of the human soul occurred in the 1950 encyclical Humani Generis, written by Pope Pius XII. Here is the relevant passage:
36. For these reasons the Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter -- for the Catholic faith obliges us to hold that souls are immediately created by God. . . .
The pope doesn't however, state exactly when this occurs. Does that mean that Catholics are still out to sea on that question? No. It was probably the case that Pius XII was presupposing this, based on past precedent. Ninety-six years earlier, Blessed Pope Pius IX, in declaring the ex cathedra doctrine of Mary's Immaculate Conception, wrote:
Accordingly, by the inspiration of the Holy Spirit, for the honor of the Holy and undivided Trinity, for the glory and adornment of the Virgin Mother of God, for the exaltation of the Catholic Faith, and for the furtherance of the Catholic religion, by the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: "We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful."
(Ineffabilis Deus, 1854)
If this occurred in the first instance of Mary's conception, then she obviously possessed a soul, since it was capable of obtaining original sin, but for the special act of grace. Therefore, this definition presupposes that human beings possess a soul from conception. If it were true of Mary, it would also be true of any human being, since she, too, is a created human being.
And again, Pope St. Pius X, wrote:
18. No, to the Christian intelligence the idea is unthinkable that the flesh of Christ, holy, stainless, innocent, was formed in the womb of Mary of a flesh which had ever, if only for the briefest moment, contracted any stain. And why so, but because an infinite opposition separates God from sin? There certainly we have the origin of the conviction common to all Christians that Jesus Christ before, clothed in human nature, He cleansed us from our sins in His blood, accorded Mary the grace and special privilege of being preserved and exempted, from the first moment of her conception, from all stain of original sin. . . .
22. But let people believe and confess that the Virgin Mary has been from the first moment of her conception preserved from all stain; . . .
(Ad Diem Illum Laetissimum / On the Immaculate Conception, 1904)
In the 1908 article, "Creationism," in the Catholic Encyclopedia, it is stated that "most neo-Scholastics hold that the rational soul is created and infused into the incipient human being at the moment of conception," and "the rational soul is infused into the organism at conception, as the modern opinion holds . . ." Likewise, the article, "Soul," from 1912, asserts: "Many modern theologians . . . maintain that a fully rational soul is infused into the embryo at the first moment of its existence."
Of course, that was prior to our modern understanding of genetics. Now we know that everything, physically, that the preborn child needs to grow to an adult, is present at the moment of conception, in the DNA. That has, no doubt, been a key factor in causing virtually all orthodox Catholic theologians to hold that this is also when the special creation of the soul occurs.
Even in the days when there was some debate on the subject, and some eminent theologians such as even St. Thomas Aquinas, due to the biological ignorance of the period, and in the school of Aristotelian philosophy and its primitive biology, thought that the soul only fully developed some period of time after conception, the Church nevertheless condemned abortion at all stages. This is blatantly contrary to the damnable lies that some self-interested, compromised pro-abortion politicians (e.g., Joe Biden and Nancy Pelosi) are now spewing, as if the Catholic Church ever doubted that abortion were sinful:
1184 34. It is permitted to bring about an abortion before the animation of the foetus, lest the girl found pregnant be killed or defamed.
1185 35. It seems probable that every foetus (as long as it is in the womb) lacks a rational soul and begins to have the same at the time that it is born; and consequently it will have to be said that no homicide is committed in any abortion.
(Pope Innocent XI, Various Errors on Moral Subjects (II) / Condemned in a decree of the Holy Office, March 4, 1679. Denzinger 1184-1185)
The Didache is one of the earliest non-biblical apostolic writings. It stated:
The second commandment in the Teaching means: Commit no murder, adultery, sodomy, fornication, or theft. Practise no magic, sorcery, abortion, or infanticide.
There was an extraordinary patristic consensus on the question:
Epistle of Barnabas: Those on the "way of darkness" include in 20.2, "the
murderers of children, aborting the work of God." (c. 132-138)Tertullian, Apologeticum 9:8: For us, since murder has been forbidden, it
is also not permitted to dissolve what is conceived in the womb while the
blood is being formed into a human being. It is an anticipation of murder
to keep one from being born; nor does it make a difference whether one
takes the life of one already born, or disturbs one in the process of being
born: even the one who is going to be a human being is one." Text from
Sources Chrétiennes No.108, p.184. (Written about 197 A.D.)
St. Cyprian of Carthage, Epistle 52, to Cornelius: "He [the schismatic
Novatian] struck the womb of his wife with his heel and hurried an
abortion, thereby causing parricide." (Written about 251 AD)
St. Basil the Great, Epistle 138: "He who destroys the fetus deliberately
is guilty of murder." PG 36:672. (Written about 375 AD)
St. Jerome, Epistle 22.13: [speaking of virgins] "Others drink for
sterility and commit murder on the human not yet sown. Some when they sense
that they have conceived by sin, consider the poisons for abortion, and
frequently die themselves along with it, and go to hell guilty of three
crimes: murdering themselves, committing adultery against Christ, and murder
against their unborn child." PL 22.401. (Written about 380 AD).
St. Ambrose, "On the Hexaemeron" 5:18: "The rich women, to avoid dividing
the inheritance among many, kill their own fetus in the womb and with
murderous juices extinguish in the genital chamber their children." PL
14:231. (Written about 386 AD).
St. John Chrysostom, "Homilies on Romans" 24: To destroy the fetus "is
something worse than murder." The one who does this "does no to take away
life that has already been born, but prevents it from being born." PG
60.626-27. (Written about 391 AD).
St. Augustine, "De nuptiis et concupiscentia" 1:15: "At times their lustful
cruelty or cruel lust goes so far as to obtain poisons to cause sterility;
and if this does not work,to somehow extinguish and destroy the fetus
conceived within the womb, wishing the offspring to be killed before living,
or if it was living in the womb, to be killed before being born." PL44:423-
24. (Written about 419 AD).
Pope Stephen V, "Epistle to Archbishop of Mainz," Sept 14, 887 (SA
670): "If he who destroys what is conceived in the womb by abortion is a
murderer, how much more is he unable to excuse himself of murder who kills
a child even one day old."
Catholic apologist Jimmy Akin, in an article on this very subject (This Rock, March 2002), clarifies as to St. Thomas Aquinas' views:
One of the arguments used by pro-abortion individuals is that it is permissible to kill an unborn child because nobody knows when the child gets a soul. Prior to this point, the unborn would not be a human being, and so killing it would not be homicide. . . .
It is sometimes claimed that Thomas Aquinas believed that the unborn did not acquire a soul until several weeks after conception. This is not true. Aquinas believed that the unborn had a soul (a rational, human soul) from the time it was conceived. However, following Aristotelian science, he (and a few other Western writers) thought that conception was an extended process that did not finish until forty or ninety days into the pregnancy: "The conception of the male finishes on the fortieth day and that of the woman on the ninetieth, as Aristotle says in the IX Book of the Animals" (Aquinas, Commentary on III Sentences 3:5:2).
Aquinas was correct that the unborn receive their souls at conception; he was merely mistaken on when conception was finished, due to the science available. As modern medicine has shown, conception in humans occurs almost instantaneously, as soon as the sperm and the ovum unite. This may occur as soon as twenty minutes after the marital act.
Aquinas and a few other medieval Western writers held the forty-to-ninety-day conception theory, but the biological discoveries of the nineteenth century proved it wrong. The view provides little comfort for abortion advocates today for a variety of reasons. It was based on primitive science. It draws a distinction between males and females that many today would regard as sexist. It was held by only a few writers. No single theologian (even Aquinas) speaks for the Church. The writers who favored the theory also opposed abortion as intrinsically evil at any stage.
When viewed without the lens of Aristotelian science, the biblical view of ensoulment becomes clear. In the Old Testament, the psalmist assumes the humanity of the unborn child at conception when he says, "Indeed, I was born guilty, a sinner when my mother conceived me" (Ps. 51:5, NRSV). This indicates that the unborn child possesses a sinful, fallen nature at the time of conception (though it does not manifest in actual, personal sins until later; cf. Romans 9:11). Since sin is a spiritual phenomenon, the presence of a sinful nature indicates a spiritual nature and thus a soul, making the child a complete human being from conception.
Akin offers further (I think, rather strong) arguments from reason as to the existence of a soul since conception:
The possession of the soul at all stages of development is also indicated by natural reason, once one understands what a soul is. From an ultimate perspective, a human is comprised of a human soul serving as the substantial form of a human body (cf. Summa Theologiae I:75:4), as indicated in Genesis 2:7. The fact that a soul is needed to turn a human body into a human has sufficiently penetrated the popular consciousness that people recognize the presence of a soul is tied to the right to life.
This leads to the argument in which pro-abortion individuals try to turn the concept of the soul against pro-lifers by arguing that there is no empirical way of determining the presence the soul, making it a matter of faith or personal opinion.
One response to this argument is to take on the concept of the soul. According to biblical theology, the soul (the spirit) is the life-principle of the body. As such, so long as a human body is alive, it has a human soul, for, James tells us, "the body apart from the spirit is dead" (Jas. 2:26). This point of biblical theology was infallibly proclaimed, using philosophical terminology, by the Council of Vienna (1311–1312). The Council dogmatically defined that the soul is the substantial form of a living human body—the metaphysical form that gives the body its humanness and its life (DS 902 [D 481], CCC 365). When the soul departs, the body ceases to be living, loses its integrity, and begins to decay.
Given this, a pro-life advocate may say that there is an empirical test for the presence of the human soul. Though the soul itself cannot be empirically observed, its presence can be detected (just as an electron itself cannot be directly observed, but the presence of an electron can be detected through various scientific means). The test is simple: If you have a living human body, it is made alive by a human soul. This reduces the issue to the question of biological humanness.
Another way to deal with the argument is to turn the abortion activist’s assertion—that the soul is undetectable—against him. One may argue that if the soul is undetectable, then its presence or absence cannot be used as a test for humanness in a secular society. People cannot be allowed to terminate the lives of others based on their individual beliefs concerning whether their victims have souls. Therefore, we must rely on what we can test, which is whether a life form is biologically human.
This approach will often be more appropriate than arguing about the presence or absence of souls, especially when one is talking with a person of little or no religious faith. It also completely undercuts the argument that the rights of the unborn are a purely religious matter.
The late great Pope John Paul II eloquently summarized Catholic moral teaching on the soul of the preborn child, his or her intrinsic value, and abortion in his extraordinary 1995 encyclical, Evangelium Vitae (I've created handy links to many of the related Church documents that are referred to):
"For you formed my inmost being" (Psalm 139:13): The dignity of the unborn child
44. Human life finds itself most vulnerable when it enters the world and when it leaves the realm of time to embark upon eternity. The word of God frequently repeats the call to show care and respect, above all where life is undermined by sickness and old age. Although there are no direct and explicit calls to protect human life at its very beginning, specifically life not yet born, and life nearing its end, this can easily be explained by the fact that the mere possibility of harming, attacking, or actually denying life in these circumstances is completely foreign to the religious and cultural way of thinking of the People of God.
In the Old Testament, sterility is dreaded as a curse, while numerous offspring are viewed as a blessing: "Sons are a heritage from the Lord, the fruit of the womb a reward" (Ps 127:3; cf. Ps 128:3-4). This belief is also based on Israel's awareness of being the people of the Covenant, called to increase in accordance with the promise made to Abraham: "Look towards heaven, and number the stars, if you are able to number them . . . so shall your descendants be" (Gen 15:5). But more than anything else, at work here is the certainty that the life which parents transmit has its origins in God. We see this attested in the many biblical passages which respectfully and lovingly speak of conception, of the forming of life in the mother's womb, of giving birth and of the intimate connection between the initial moment of life and the action of God the Creator.
"Before I formed you in the womb I knew you, and before you were born I consecrated you" (Jer 1:5): the life of every individual, from its very beginning, is part of God's plan. Job, from the depth of his pain, stops to contemplate the work of God who miraculously formed his body in his mother's womb. Here he finds reason for trust, and he expresses his belief that there is a divine plan for his life: "You have fashioned and made me; will you then turn and destroy me? Remember that you have made me of clay; and will you turn me to dust again? Did you not pour me out like milk and curdle me like cheese? You clothed me with skin and flesh, and knit me together with bones and sinews. You have granted me life and steadfast love; and your care has preserved my spirit" (Job 10:8-12). Expressions of awe and wonder at God's intervention in the life of a child in its mother's womb occur again and again in the Psalms.
How can anyone think that even a single moment of this marvellous process of the unfolding of life could be separated from the wise and loving work of the Creator, and left prey to human caprice? Certainly the mother of the seven brothers did not think so; she professes her faith in God, both the source and guarantee of life from its very conception, and the foundation of the hope of new life beyond death: "I do not know how you came into being in my womb. It was not I who gave you life and breath, nor I who set in order the elements within each of you. Therefore the Creator of the world, who shaped the beginning of man and devised the origin of all things, will in his mercy give life and breath back to you again, since you now forget yourselves for the sake of his laws" (2 Mac 7:22-23).
45. The New Testament revelation confirms the indisputable recognition of the value of life from its very beginning. The exaltation of fruitfulness and the eager expectation of life resound in the words with which Elizabeth rejoices in her pregnancy: "The Lord has looked on me. . . to take away my reproach among men" (Lk 1:25). And even more so, the value of the person from the moment of conception is celebrated in the meeting between the Virgin Mary and Elizabeth, and between the two children whom they are carrying in the womb. It is precisely the children who reveal the advent of the Messianic age: in their meeting, the redemptive power of the presence of the Son of God among men first becomes operative. As Saint Ambrose writes: "The arrival of Mary and the blessings of the Lord's presence are also speedily declared. . . Elizabeth was the first to hear the voice; but John was the first to experience grace. She heard according to the order of nature; he leaped because of the mystery. She recognized the arrival of Mary; he the arrival of the Lord. The woman recognized the woman's arrival; the child, that of the child. The women speak of grace; the babies make it effective from within to the advantage of their mothers who, by a double miracle, prophesy under the inspiration of their children. The infant leaped, the mother was filled with the Spirit. The mother was not filled before the son, but after the son was filled with the Holy Spirit, he filled his mother too".
[ . . . ]
"Your eyes beheld my unformed substance" (Psalm 139:16): The unspeakable crime of abortion
58. Among all the crimes which can be committed against life, procured abortion has characteristics making it particularly serious and deplorable. The Second Vatican Council defines abortion, together with infanticide, as an "unspeakable crime".
But today, in many people's consciences, the perception of its gravity has become progressively obscured. The acceptance of abortion in the popular mind, in behaviour and even in law itself, is a telling sign of an extremely dangerous crisis of the moral sense, which is becoming more and more incapable of distinguishing between good and evil, even when the fundamental right to life is at stake. Given such a grave situation, we need now more than ever to have the courage to look the truth in the eye and to call things by their proper name, without yielding to convenient compromises or to the temptation of self-deception. In this regard the reproach of the Prophet is extremely straightforward: "Woe to those who call evil good and good evil, who put darkness for light and light for darkness" (Is 5:20). Especially in the case of abortion there is a widespread use of ambiguous terminology, such as "interruption of pregnancy", which tends to hide abortion's true nature and to attenuate its seriousness in public opinion. Perhaps this linguistic phenomenon is itself a symptom of an uneasiness of conscience. But no word has the power to change the reality of things: procured abortion is the deliberate and direct killing, by whatever means it is carried out, of a human being in the initial phase of his or her existence, extending from conception to birth.
The moral gravity of procured abortion is apparent in all its truth if we recognize that we are dealing with murder and, in particular, when we consider the specific elements involved. The one eliminated is a human being at the very beginning of life. No one more absolutely innocent could be imagined. In no way could this human being ever be considered an aggressor, much less an unjust aggressor! He or she is weak, defenceless, even to the point of lacking that minimal form of defence consisting in the poignant power of a newborn baby's cries and tears. The unborn child is totally entrusted to the protection and care of the woman carrying him or her in the womb. And yet sometimes it is precisely the mother herself who makes the decision and asks for the child to be eliminated, and who then goes about having it done.
It is true that the decision to have an abortion is often tragic and painful for the mother, insofar as the decision to rid herself of the fruit of conception is not made for purely selfish reasons or out of convenience, but out of a desire to protect certain important values such as her own health or a decent standard of living for the other members of the family. Sometimes it is feared that the child to be born would live in such conditions that it would be better if the birth did not take place. Nevertheless, these reasons and others like them, however serious and tragic, can never justify the deliberate killing of an innocent human being.
59. As well as the mother, there are often other people too who decide upon the death of the child in the womb. In the first place, the father of the child may be to blame, not only when he directly pressures the woman to have an abortion, but also when he indirectly encourages such a decision on her part by leaving her alone to face the problems of pregnancy: in this way the family is thus mortally wounded and profaned in its nature as a community of love and in its vocation to be the "sanctuary of life". Nor can one overlook the pressures which sometimes come from the wider family circle and from friends. Sometimes the woman is subjected to such strong pressure that she feels psychologically forced to have an abortion: certainly in this case moral responsibility lies particularly with those who have directly or indirectly obliged her to have an abortion. Doctors and nurses are also responsible, when they place at the service of death skills which were acquired for promoting life.
But responsibility likewise falls on the legislators who have promoted and approved abortion laws, and, to the extent that they have a say in the matter, on the administrators of the health-care centres where abortions are performed. A general and no less serious responsibility lies with those who have encouraged the spread of an attitude of sexual permissiveness and a lack of esteem for motherhood, and with those who should have ensured--but did not--effective family and social policies in support of families, especially larger families and those with particular financial and educational needs. Finally, one cannot overlook the network of complicity which reaches out to include international institutions, foundations and associations which systematically campaign for the legalization and spread of abortion in the world. In this sense abortion goes beyond the responsibility of individuals and beyond the harm done to them, and takes on a distinctly social dimension. It is a most serious wound inflicted on society and its culture by the very people who ought to be society's promoters and defenders. As I wrote in my Letter to Families, "we are facing an immense threat to life: not only to the life of individuals but also to that of civilization itself". We are facing what can be called a "structure of sin" which opposes human life not yet born.
60. Some people try to justify abortion by claiming that the result of conception, at least up to a certain number of days, cannot yet be considered a personal human life. But in fact, "from the time that the ovum is fertilized, a life is begun which is neither that of the father nor the mother; it is rather the life of a new human being with his own growth. It would never be made human if it were not human already. This has always been clear, and. . . modern genetic science offers clear confirmation. It has demonstrated that from the first instant there is established the programme of what this living being will be: a person, this individual person with his characteristic aspects already well determined. Right from fertilization the adventure of a human life begins, and each of its capacities requires time--a rather lengthy time--to find its place and to be in a position to act".
Even if the presence of a spiritual soul cannot be ascertained by empirical data, the results themselves of scientific research on the human embryo provide "a valuable indication for discerning by the use of reason a personal presence at the moment of the first appearance of a human life: how could a human individual not be a human person?".
 CONGREGATION FOR THE DOCTRINE OF THE FAITH, Instruction on Respect for Human Life in its Origin and on the Dignity of Procreation Donum Vitae (22 February 1987), I, No. 1: AAS 80 (1988), 78-79.
Furthermore, what is at stake is so important that, from the standpoint of moral obligation, the mere probability that a human person is involved would suffice to justify an absolutely clear prohibition of any intervention aimed at killing a human embryo. Precisely for this reason, over and above all scientific debates and those philosophical affirmations to which the Magisterium has not expressly committed itself, the Church has always taught and continues to teach that the result of human procreation, from the first moment of its existence, must be guaranteed that unconditional respect which is morally due to the human being in his or her totality and unity as body and spirit: "The human being is to be respected and treated as a person from the moment of conception; and therefore from that same moment his rights as a person must be recognized, among which in the first place is the inviolable right of every innocent human being to life".
 Ibid., loc. cit., 79.
61. The texts of Sacred Scripture never address the question of deliberate abortion and so do not directly and specifically condemn it. But they show such great respect for the human being in the mother's womb that they require as a logical consequence that God's commandment "You shall not kill" be extended to the unborn child as well.
Human life is sacred and inviolable at every moment of existence, including the initial phase which precedes birth. All human beings, from their mothers' womb, belong to God who searches them and knows them, who forms them and knits them together with his own hands, who gazes on them when they are tiny shapeless embryos and already sees in them the adults of tomorrow whose days are numbered and whose vocation is even now written in the "book of life" (cf. Ps 139: 1, 13-16). There too, when they are still in their mothers' womb--as many passages of the Bible bear witness
 Hence the Prophet Jeremiah: "The word of the Lord came to me saying: 'Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations"' (1:4-5). The Psalmist, for his part, addresses the Lord in these words: "Upon you I have leaned from my birth; you are he who took me from my mother's womb" (Ps 71:6; cf. Is 46:3; Job 10:8-12; Ps 22:10-11). So too the Evangelist Luke in the magnificent episode of the meeting of the two mothers, Elizabeth and Mary, and their two sons, John the Baptist and Jesus, still hidden in their mothers' wombs (cf. 1:39-45) emphasizes how even before their birth the two little ones are able to communicate: the child recognizes the coming of the Child and leaps for joy.
--they are the personal objects of God's loving and fatherly providence.
Christian Tradition--as the Declaration issued by the Congregation for the Doctrine of the Faith points out so well
 Cf. Declaration on Procured Abortion (18 November 1974), No. 7: AAS 66 (1974), 740-747.
--is clear and unanimous, from the beginning up to our own day, in describing abortion as a particularly grave moral disorder. From its first contacts with the Greco-Roman world, where abortion and infanticide were widely practised, the first Christian community, by its teaching and practice, radically opposed the customs rampant in that society, as is clearly shown by the Didache mentioned earlier.
 "You shall not kill a child by abortion nor shall you kill it once it is born": V, 2: Patres Apostolici, ed. F.X. Funk, I, 17.
Among the Greek ecclesiastical writers, Athenagoras records that Christians consider as murderesses women who have recourse to abortifacient medicines, because children, even if they are still in their mother's womb, "are already under the protection of Divine Providence".
 Apologia on behalf of the Christians, 35: PG 6, 969.
Among the Latin authors, Tertullian affirms: "It is anticipated murder to prevent someone from being born; it makes little difference whether one kills a soul already born or puts it to death at birth. He who will one day be a man is a man already".
 Apologeticum, IX, 8: CSEL 69, 24.
Throughout Christianity's two thousand year history, this same doctrine has been constantly taught by the Fathers of the Church and by her Pastors and Doctors. Even scientific and philosophical discussions about the precise moment of the infusion of the spiritual soul have never given rise to any hesitation about the moral condemnation of abortion.
62. The more recent Papal Magisterium has vigorously reaffirmed this common doctrine. Pius XI in particular, in his Encyclical Casti Connubii, rejected the specious justifications of abortion.
Pius XII excluded all direct abortion, i.e., every act tending directly to destroy human life in the womb "whether such destruction is intended as an end or only as a means to an end".
 Address to the Biomedical Association "San Luca" (12 November 1944): Discorsi e Radiomessaggi, VI (1944-1945), 191; cf. Address to the Italian Catholic Union of Midwives (29 October 1951), No. 2: AAS 43 (1951), 838.
John XXIII reaffirmed that human life is sacred because "from its very beginning it directly involves God's creative activity".
The Second Vatican Council, as mentioned earlier, sternly condemned abortion: "From the moment of its conception life must be guarded with the greatest care, while abortion and infanticide are unspeakable crimes".
 Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 51.
The Church's canonical discipline, from the earliest centuries, has inflicted penal sanctions on those guilty of abortion. This practice, with more or less severe penalties, has been confirmed in various periods of history. The 1917 Code of Canon Law punished abortion with excommunication.
 Canon 2350, # 1.
The revised canonical legislation continues this tradition when it decrees that "a person who actually procures an abortion incurs automatic (latae sententiae) excommunication".
The excommunication affects all those who commit this crime with knowledge of the penalty attached, and thus includes those accomplices without whose help the crime would not have been committed.
 Cf. ibid., canon 1329; also Code of Canons of the Eastern Churches, canon 1417.
By this reiterated sanction, the Church makes clear that abortion is a most serious and dangerous crime, thereby encouraging those who commit it to seek without delay the path of conversion. In the Church the purpose of the penalty of excommunication is to make an individual fully aware of the gravity of a certain sin and then to foster genuine conversion and repentance.
Given such unanimity in the doctrinal and disciplinary tradition of the Church, Paul VI was able to declare that this tradition is unchanged and unchangeable.
Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops--who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine--I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium.
 Cf. SECOND VATICAN ECUMENICAL COUNCIL, Dogmatic Constitution on the Church Lumen Gentium, 25.
No circumstance, no purpose, no law whatsoever can ever make licit an act which is intrinsically illicit, since it is contrary to the Law of God which is written in every human heart, knowable by reason itself, and proclaimed by the Church.