See the introductory post, New Upcoming Project: Refutation of William Whitaker's Disputation on Holy Scripture on Sola Scriptura.
I am utilizing a copy of the book available at Internet Archive.
Whitaker's words will be in blue. Page numbers will correspond to the above book version.
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There is no consequential force then in the argument, that because he says "which I command," not, "which I have written," therefore this word was not committed to writing, but delivered by oral tradition. Besides, if Moses had entrusted some things orally to certain persons, which he considered unfit to be written; to whom could he have committed them rather than to Joshua, to whom he imparted all his counsels, and who was his successor in office? . . . when God forbids them to add, he signifies that this body of doctrine was so perfect as that nothing could or should be added to it; and that, therefore, we should acquiesce in it, be satisfied with it, and cleave to it alone. They add, therefore, who determine that this teaching is not complete and full. And when we shew that this word is written, we shew that the written word contains a full and perfect body of doctrine, to which nothing should be added. The ancient Jews understood and explained these words to mean that nothing should be added to the written word. (p. 617)
In fact, the Jews believed that Moses received an oral law on Mt. Sinai, in addition to the written Torah. This was the mainstream position, which was held by the Pharisees, and denied by the Sadducees, and it was followed by Jesus and the early Christians. I have provided nine biblical proofs that this is the case in my paper, Biblical Evidence for the Oral Torah (Hence, by Analogy, Oral Apostolic Tradition).
Jesus even utilized oral tradition, as codified in the Talmud, in His Sermon on the Mount. If in fact it is the case that oral tradition was considered authoritative by Jesus and the first Christians, then Whitaker's contentions about "written-only" and sola Scriptura are so much hooey. We are to follow the example of our Lord and the apostles, not unbiblical traditions of men that contradict them. Whitaker comments on the following Bible passage:
Galatians 1:8 (RSV) But even if we, or an angel from heaven, should preach to you a gospel contrary to that which we preached to you, let him be accursed.
. . . if an angel from heaven preach to you anything besides what you have received in the scriptures of the law and of the gospel, let him be accursed!" In these words we should observe and consider the following points: First, that all that Paul taught may be found in the scriptures. . . . whatever is preached or announced besides what is contained in these scriptures, is to be wholly rejected. His words are, "besides what is written:" therefore not only that which is contrary to, but that also which is beside the scriptures, should be refused. . . . Paul is here speaking of the scriptures, and condemning every doctrine not therein delivered . . . It appears therefore hence, that whatever is beside the scriptures, is alien from them, and therefore should be rejected. (pp. 624-625)
Whitaker is contending that Paul condemns anything (i.e., tradition) "besides" the gospel, which is contained in its entirety (as we agree) in Scripture. But this is not what the text says. "Contrary" to the gospel, doesn't necessarily preclude traditions that are not expressly laid out in Scripture, as Whitaker conceded (at least partially) in the citation from p. 625 (next one below): since traditions may indeed be fully in accord with the gospel. In other words, if this is used as an argument against tradition per se, it fails. Paul's statement is only an argument against doctrines contrary to the gospel. It is not an "unwrittten" (tradition) vs. "written" (Bible / gospel in the Bible) dichotomy but rather, a "gospel" vs. "contrary to the gospel" dichotomy.
. . . Traditions are either consonant to scripture, and then they should be received, and those who do not receive them are condemned in these words; or they are, as Basil expresses it, alien from scripture, and then they should be rejected. These fathers speak of those traditions which are consonant to scripture, not of such as are alien from it. (p. 625)
Amen! This is what Catholics have been saying all along: our traditions (apostolic traditions) are completely in harmony with Scripture and are indicated there either explicitly, implicitly, or straightforwardly deduced from the same sort of evidences. If Whitaker goes this far in espousing traditions, then he shoots himself in the foot insofar as he wishes to defend sola Scriptura. He concedes too much. He also fails to grasp that the Catholic Church holds to exactly this, so that he is unaware of the position of his opponent. But, having stumbled upon the truth in this instance, Whitaker immediately qualifies (or contradicts?) it, in replying to Bellarmine's argument, which he summarizes:
He says, in the second place, that the word "beside" in this place is equivalent to "against:" so as that Paul here anathematizes those who deliver anything against, not beside, the scriptures; consequently, that new doctrines are not here prohibited, provided they do not contradict the scriptures. . . . We, however, take the word "beside" in its strict sense, so as to bring under this denunciation whatever is delivered beside that gospel delivered by the apostle. (p. 625)
This is an absurd position because it massively contradicts Paul's frequent reference to (in a positive sense) apostolic tradition and traditions, and oral teaching (1 Cor 11:2; 2 Thess 2:15; 3:6; Phil 4:9; Col 2:8; 2 Tim 1:13-14; 2:2). Whitaker interprets the word rendered "contrary to" in RSV and "besides" in his own translation as "anything besides the gospel" rather than "anything contrary to the gospel." He moves the emphasis from being contrary, to being simply other. Whitaker keeps up his odd and, I think, implausible (but interesting and clever) textual argument (italics added to highlight the words under consideration):
But the apostle does not use the term against, because the false apostles would have denied that it was against that gospel which Paul himself had delivered. In order, therefore, to obviate this false pretence, the apostle says, "beside what I preached unto you, and ye received;" as if he had said, I taught you nothing of the kind; therefore those who introduce such things are to be avoided, and by no means to be listened to. Thus it is certain that beside suits the apostle's design much better than against. (p. 626)
Alright; we have heard Whitaker's opinion on how it ought to be translated, to best convey Paul's meaning in Galatians 1:8. Now let's see what Bible translators think, in order to best determine or ascertain exactly what Paul intended. None use "besides." Several use "other." I have placed those in Whitaker's favor, as a synonym of "besides," even though it is quite arguable that "other" is closer to "contradicts" (or similar concept) than "besides". To say "other than" or "another so-and-so" is usually (though not necessarily) to indicate a contradiction or a contrary position.
If I say, "the Jehovah's Witnesses believe in a doctrine other than trinitarianism," they obviously contradict trinitarianism. Since there is only one gospel, "another" or "any other" gospel obviously contradicts it; if it didn't, it would be the same in the first place. Thus, Whitaker's emphasis on this textual argument is mostly a distinction without a difference; much ado about nothing. Nevertheless, let's see how many (non-Catholic) translations opted for the more directly, explicitly "contradictory" sense, over against Whitaker's position (that he seems quite irrationally dogmatic about):
Versions Opposed to Whitaker's Interpretation 
RSV . . . a gospel contrary to that which we preached to you . . .
NEB . . . a gospel at variance with . . .
NASB . . . a gospel contrary to . . .
Williams . . . a good news that is contrary . . .
Goodspeed . . . good news that contradicts . . .
Moffatt . . . a gospel that contradicts . . .
Amplified . . . a gospel contrary to and different . . .
Wuest . . . a gospel . . . which goes beyond that which you took so eagerly . . .
Barclay . . . a gospel . . . which is at variance with . . .
CEV . . . anything different from our message . . .
TEV . . . a gospel that is different . . .
Versions in Favor of Whitaker's Interpretation 
KJV / Phillips . . . any other gospel . . .
NKJV / Lamsa . . . any other gospel . . .
NIV / REB / ASV . . . a gospel other than . . .
Beck . . . any other good news . . .
Living . . . any other message . . .
The tally is 11-9 against Whitaker's interpretation: no slam-dunk for his side. And granting my remarks about the meaning of "other", arguably he has little or no case at all. In any event, he certainly has no warrant to be dogmatic based on this text (and one word) alone. He goes on to argue in the same manner, regarding a similar passage:
Romans 16:17 (RSV) I appeal to you, brethren, to take note of those who create dissensions and difficulties, in opposition to the doctrine which you have been taught; avoid them.
I confess it, and so does Beza: for whatever is against scripture is also beside it; and, conversely also, whatever in our holy religion is taught beside the scriptures, is against the scriptures too, if it carry with it any notion of necessity, that is, if it be proposed as a necessary doctrine. Since the apostles delivered abundantly all necessary things in the scriptures, whatever is urged as necessary beside the scriptures is justly deemed contrary to them. I confess that Trapd may sometimes be conveniently translated against, but not in this place. . . . in religious matters beside is equivalent to against the scriptures: but we have already shewn the reason why the apostle uses the term beside rather than against, because it suited his purpose better. (pp. 626-627; italics added for the references to particular words in the biblical text)
We agree that all that is necessary in Christian doctrine is in Scripture (this is material sufficiency): just not always explicitly so: as Whitaker and most Protestants tend to irrationally require. But he has taken his argument to the extreme again, foolishly insisting on a certain translation, when there is plenty of room for different, honestly-held opinions. Let's do a tally again, with the same twenty translations. At least last time it was "close"; this time it is unanimous against Whitaker's dogmatic take on the passage:
Versions Opposed to Whitaker's Interpretation 
RSV / Amplified . . . opposition to the doctrine . . .
NEB / NASB / Wuest / NIV / REB . . . contrary to the teaching . . .
Williams / Goodspeed . . . opposition to the instruction . . .
Moffatt / KJV / MKJV / Lamsa / ASV . . . contrary to the doctrine . . .
Barclay . . . in defiance of the teaching . . .
CEV . . . refusing to do what all of you were taught . . .
TEV . . . against the teaching . . .
Phillips . . . plain opposition to the teaching . . .
Beck . . .going against the teaching . . .
Living . . .contrary to what you have been taught . . .
Versions in Favor of Whitaker's Interpretation [none]
. . . it is certain that Chrysostom maintains the perfection of scripture, and is on our side against the papists: for in these words he subverts both the Jesuit's answers, since he determines that the apostle both speaks of the written word of God, and condemns whatever is preached not only against, but beside the scriptures. (p. 627)
St. John Chrysostom was only asserting the material sufficiency of Scripture in the passage that Whitaker seized upon (not sola Scriptura). But of course (as usual) he ignored the many places in the same saint's writing where he explicitly espouses the authority of extra-biblical tradition. I have documented these in my book, The Church Fathers Were Catholic:
[T]here was much also that was not written. Like that which is written, the unwritten too is worthy of belief. So let us regard the Tradition of the Church also as worthy of belief. (Homilies on 2 Thess 4:2; commenting on 2 Thess 2:15)
"That ye remember me in all things, and hold fast the traditions, even as I delivered them to you." It appears then that he used at that time to deliver many things also not in writing, which he shows too in many other places. But at that time he only delivered them, whereas now he adds an explanation of their reason: thus both rendering the one sort, the obedient, more steadfast, and pulling down the others' pride, who oppose themselves. (Homily XXVI on 1 Corinthians; commenting on 1 Cor 11:2; NPNF 1, Vol. XII)
"So then, brethren, stand fast, and hold the traditions which ye were taught, whether by word, or by Epistle of ours." Hence it is manifest, that they did not deliver all things by Epistle, but many things also unwritten, and in like manner both the one and the other are worthy of credit. Therefore let us think the tradition of the Church also worthy of credit. It is a tradition, seek no farther. Here he shows that there were many who were shaken. (On Second Thessalonians, Homily IV)
Not by letters alone did Paul instruct his disciple in his duty, but before by words also which he shows, both in many other passages, as where he says, “whether by word or our Epistle” (2 Thess. ii. 15.), and especially here. Let us not therefore suppose that anything relating to doctrine was spoken imperfectly. For many things he delivered to him without writing. Of these therefore he reminds him, when he says, “Hold fast the form of sound words, which thou hast heard of me.” (Homily III on 2 Timothy - on 2 Tim 1:13-18; NPNF 1, Vol. XIII)
[I]t was no object with them to be writers of books: in fact, there are many things which they have delivered by unwritten tradition. (On Acts of the Apostles, Homily 1; NPNF 1, Vol. XI)
Ver. 8. “Now as Jannes and Jambres withstood Moses, so do these also resist the truth.” Who are these? The magicians in the time of Moses. But how is it their names are nowhere else introduced? Either they were handed down by tradition, or it is probable that Paul knew them by inspiration. (Homily VIII on 2 Timothy; NPNF 1, Vol. XIII)
For, “remember,” he says, “the words of the Lord which he spake: It is more blessed to give than to receive.” (v. 35.) And where said He this? Perhaps the Apostles delivered it by unwritten tradition; or else it is plain from (recorded sayings, from) which one could infer it. (Homily XLV on Acts 20:32; NPNF 1, Vol. XIII)
Now the fourth passage of scripture which we cite against traditions is contained in the last verse of the twentieth chapter of John, . . . (p. 628)
John 20:31 but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.
It is manifest from these words, that all necessary things may be found in those which are written, because by these a full and perfect faith may be produced, inasmuch as such a faith is capable of procuring eternal life. (p. 628)
It doesn't follow logically, from the statement, "written document x produces faith and salvation", that the statement, "only written document x produces faith and salvation" is also true. Whitaker has falsely assumed that the second proposition is 1) stated in the passage, and 2) if not stated, necessarily flows from the first. It does not. And in fact, this is asserted repeatedly in Scripture itself.
Most (possibly all) recorded conversions or new professions of faith or baptisms in the New Testament came as a result of preaching (or sometimes signs), not a written text (see, e.g., Lk 8:12; Acts 4:4; 8:12; 10:44; 11:14; Rom 10:14, 17; 1 Cor 1:21; 15:11; Eph 1:13; 1 Thess 2:16), so it is foolish to assert that only the written word can produce this result. This is obvious today as well. Someone could observe a Christian and be so moved, as to want to believe as he or she does. This is stated in 1 Peter 3:1: ". . . some, though they do not obey the word, may be won without a word by the behavior of their wives,". St. Paul states something very similar:
1 Corinthians 10:33 just as I try to please all men in everything I do, not seeking my own advantage, but that of many, that they may be saved.
2 Corinthians 3:2-3 You yourselves are our letter of recommendation, written on your hearts, to be known and read by all men;  and you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
St. Paul strongly implies that men can be saved without hearing the gospel (analogy to "the law") at all, in Romans 2:11-16. There are many ways of obtaining salvation, therefore, beside through the medium of the written word of Scripture. Paul says, for example, that a woman having a child may save her soul primarily by that action (1 Tim 2:15). Works (and not faith, let alone Bible-reading) are the central criteria for entrance into heaven at the judgment (see 50 passages that assert this).
Although the evangelist does mention miracles in the preceding verse, yet the word Tavra, which he subjoins in this, is to be understood of doctrine rather than of miracles. (p. 628)
John 20:30 reads: "Now Jesus did many other signs in the presence of the disciples, which are not written in this book;". Certainly some of these signs produced new believers and disciples. I have no idea what he means or what he is referring to when he claims that the passage is also or mostly about "doctrine", but it sure sounds like special pleading.
For miracles do not properly produce faith in us, but rather confirm and support it when it hath been produced, and miracles minister to and win credence for the doctrine. (p. 628)
This is not untrue as a generality (it is often or usually the case); yet there is a passage in Scripture such as John 2:23: ". . . many believed in his name when they saw the signs which he did". This implies that the miracle or sign directly led to proclamations of faith in Jesus as His disciples. There are more such passages:
John 7:31 Yet many of the people believed in him; they said, "When the Christ appears, will he do more signs than this man has done?"
John 11:4 Many of the Jews therefore, who had come with Mary and had seen what he did [raised Lazarus from the dead], believed in him;
John 20:29 Jesus said to him, "Have you believed because you have seen me? Blessed are those who have not seen and yet believe."
. . . Scripture is not only one of those means which relate to salvation, but the entire and sole medium, the perfect and complete medium, because it produces a perfect faith. For that faith which brings salvation is perfect; and consequently the medium whereby that faith is produced is also perfect. (p. 629)
But we have seen again and again in Scripture itself that this is not true. I have followed what the Bible itself teaches, all through these critiques of Whitaker, whereas we have seen that he often invents (in his eternal struggle against the truth of the "papists") non-biblical traditions that cannot be found there, and foolishly pretends that they are scriptural teachings. Merely stating a thing does not make it so.
Whitaker's views here border on bibliolatry: making the Bible an idol and more important to salvation than Jesus' work on the cross on our behalf. Our Lord Jesus Christ is our Savior and Redeemer, not the Bible. God's written revelation, inspired and wonderful and unique as it is, is not God Himself. Words in and of themselves (even biblical ones) do not save us; they merely convey the gospel of Jesus' death on the cross, which saves us by God's grace and power. It's like saying that the tube that carries blood saves the person who receives a blood transfusion, rather than the blood itself.