Isaiah 1 provides a case study in taking passages out of context, to bolster up preconceived notions. Ironically, it was a longtime Baptist friend of mine, on the phone last night, who called my attention to this. He's good Protestant (in no "danger" of becoming Catholic whatsoever), but unlike many Protestants, doesn't try to systematically omit the importance of good works in the Christian life. He doesn't separate faith from works, as James and Paul in many places urge us not to do. He's right. This is the biblical teaching.
The passage usually cited by Protestants is Isaiah 1:18 (RSV, as throughout, when I cite Scripture):
Come now, let us reason together,
says the LORD:
though your sins are like scarlet,
they shall be as white as snow;
though they are red like crimson,
they shall become like wool.
This is reminiscent of Psalms 51:2, 7, 9-10 and King David's repentance:
Wash me thoroughly from my iniquity,
and cleanse me from my sin! . . .
 Purge me with hyssop, and I shall be clean;
wash me, and I shall be whiter than snow. . . .
 Hide thy face from my sins,
and blot out all my iniquities.
 Create in me a clean heart, O God,
and put a new and right spirit within me.
Protestants (of a certain sort) don't and won't hesitate to cite the Old Testament as authoritative if they think a passage supports their theology. When it doesn't, then the tendency is to dismiss it as irrelevant, because, well, it is the Old Testament. Quite often (if not, typically), evangelical and Reformed and fundamentalist Protestants cite Isaiah 1:18 in isolation as a prooftext for one-time justifiction and/or instant salvation. Here are some examples:
What will happen when you repent and believe? God will forgive your sins, as He said, "I, even I, am the one who wipes out your transgressions for My own sake, And I will not remember your sins." (Isaiah 43:25) and He also says, "Come now, and let us reason together," Says the LORD, " Though your sins are as scarlet, They will be as white as snow; Though they are red like crimson, They will be like wool.” (Isaiah 1:18) . . .
Now if you have repented and believe in Yeshua, you are saved and have been given His Holy Spirit who will always be with you, . . .
("When you die . . . are you ready?," Hallsville Baptist Church; my bolding)
In the Book of Isaiah the concept of [Protestant] justification is stated beautifully - Isaiah 1:18.
. . . not that pardon of sin takes sin out of the hearts and natures of men, nor changes the nature of sin, or causes it to cease to be sin; but this is to be understood of the persons of sinners, who hereby are made so white, yea, whiter than this, (Psalms 51:1) as they are considered in Christ, washed in his blood, and clothed with his righteousness, which is fine linen, clean and white; God, seeing no iniquity in them, has thus graciously dealt with them, and they being without fault, spot, or wrinkle, or any such thing.
(John Gill's Exposition of the Bible)
None of these comments seem to give the slightest attention to context. When we do that, we see that works are part and parcel of what is being dealt with. The immediate context is most striking (and jolting for those who hold to Protestant soteriology). Here are the two verses preceding Isaiah 1:18:
 Wash yourselves; make yourselves clean;
remove the evil of your doings
from before my eyes;
cease to do evil,
 learn to do good;
defend the fatherless,
plead for the widow. (cf. 1:23)
Isn't it fascinating how God, through His prophet, includes the actions of the penitent in the whole equation? Protestants tell us nothing can be done by man prior to justification (what many of them equate with a "salvation" that can't ever be lost once truly granted). We actually agree with them, insofar as we are talking about initial justification or regeneration. Those are entirely works of God's grace, and this is the clear teaching of the Council of Trent.
But then, the context of this passage doesn't fit into that scenario. Here, man is clearly doing something: quite a bit: and it can't be separated from God's pardon. Catholics simply say that it may be an instance of justification after the time of initial justification, because we don't see justification as a one-time thing (see my paper, Justification is Not by Faith Alone (Romans 4 + James 2) and is Ongoing, as Seen in Abraham's Multiple Justifications). Protestants will have to offer some other explanation concerning the context, or cease using Isaiah 1:18 as a prooftext for justification (as they define it).
God says, "Wash yourselves; make yourselves clean" and two verses later, we see the result: "your sins . . . shall be as white as snow." But Protestants want to ignore all of the actions of men in the overall passage. All of a sudden in Isaiah 1:17, God is talking about a bunch of works again! "Good," "justice," battling "oppression," helping fatherless children and widows . . . How reminiscent this is of the judgment passages, where Jesus says that the key to salvation is not faith alone, but rather (you guessed it), works:
Matthew 25:34-36 Then the King will say to those at his right hand, `Come, O blessed of my Father, inherit the kingdom prepared for you from the foundation of the world;  for I was hungry and you gave me food, I was thirsty and you gave me drink, I was a stranger and you welcomed me,  I was naked and you clothed me, I was sick and you visited me, I was in prison and you came to me.'
The same thing (works and obedience) is seen in the two passages following 1:18:
If you are willing and obedient,
you shall eat the good of the land;
 But if you refuse and rebel,
you shall be devoured by the sword;
for the mouth of the LORD has spoken.
Everything is conditional ("if . . ."). It's the furthest thing from an irrevocable unconditional promise. The entire chapter is about the nation of Israel, but generally such passages are regarded as having a double application to the Christian believer (as God's "chosen," etc.). How about Isaiah 1:27? Does it talk about faith alone as the prerequisite of justification and one-time salvation? Hardly:
Zion shall be redeemed by justice,
and those in her who repent, by righteousness.
Redeemed by "justice"? Hmmmm. Why that is salvation by works! That can't be! "Redeem" and "redemption" are good Protestant words, and they refer to God's actions only, not our own. But there it is in front of our face. Works, works, works, actions of men, cooperation with God, obedience, "working out your own salvation,": all of that terrible, idolatrous Catholic supposed "works by salvation," semi-Pelagian stuff.
It's the same everywhere in the chapter. There is no respite for the Protestant who dares to read the whole thing, and to interpret 1:18 in context, rather than atomistically isolated: as if it were merely a saying on a poster, to be repeated without any examination.
 Ah, sinful nation,
a people laden with iniquity,
offspring of evildoers,
sons who deal corruptly!
They have forsaken the LORD,
they have despised the Holy One of Israel,
they are utterly estranged.
If Israel represents the individual sinner or the Christian, here we have a nation (by double application, person) that once knew the Lord, but now no longer does. You can't "forsake" something or someone without having formerly followed them. Yet by Calvinist and Baptist and evangelical "perseverance" and "eternal security" thinking, this is not possible. One can't fall away. Grace is irresistible and election is unconditional. Thus we have to choose between what the Bible teaches and what men teach, in contradiction of it.
Under Catholic principles, on the other hand, no problem at all! Men can fall away from grace, and be restored to it through repentance and absolution and additional justification. Our view is perfectly consistent with what we find here. No special pleading or rationalization necessary; no need to force our prior view into the text in a hackneyed, arbitrary, implausible fashion (what is called eisegesis).
Prayer and worship and rituals and calling God for salvation are worthless unless we repent from the heart and indicate it by our good works:
Bring no more vain offerings;
incense is an abomination to me.
New moon and sabbath and the calling of assemblies --
I cannot endure iniquity and solemn assembly.
 Your new moons and your appointed feasts
my soul hates;
they have become a burden to me,
I am weary of bearing them.
 When you spread forth your hands,
I will hide my eyes from you;
even though you make many prayers,
I will not listen;
your hands are full of blood.
Here as everywhere in Scripture, faith and works are together. Man cooperates with God after receiving initial justification and regeneration (which come entirely by God's grace). So next time you see a Protestant prooftext for anything, be sure to check out the context. It's always a good and helpful policy to abide by.
See my related papers:
St. Paul's Teaching on the Organic Relationship of Grace / Faith and Works / Action / Obedience (Collection of 50 Pauline Passages)
Final Judgment in Scripture is Always Associated With Works And Never With Faith Alone (50 Passages)
The Interpretation and Exegesis of Romans 2-4 (Justification and Works of the Law) (Includes Very Extensive Patristic Commentary and Definitional Citations from three Protestant Bible Dictionaries)
The "Obedience of Faith" in Paul and its Soteriological Implications (Justification and Denial of "Faith Alone") [from Ferdinand Prat, S. J.; Facebook]
Dialogue on Justification in James
Biblical Evidence for the Nature of Saving Faith (Including Assent, Trust, Hope, Works, Obedience, and Sanctification)
Biblical Evidence for "Power" as a Proof and Manifestation of Infused (Catholic) Justification
Martin Luther: Strong Elements in His Thinking of Theosis and Transformational Sanctification Closely Allied with Justification
Is Christ's Righteousness Imputed to Believers?: Catholic vs. Reformed Protestant Understanding ("Adomnan" vs. John Bugay)
Catholic-Lutheran Dialogue on Justification and "Faith Alone" ("Adomnan" vs. Nathan Rinne)
"Work Out Your Own Salvation With Fear and Trembling" (Philippians 2:12): Does It Harmonize With Protestant Soteriology? (vs. Ken Temple)
Martin Luther Despised the Widespread Antinomian Distortions of His Teaching on Faith Alone and Did Not Reject Mosaic Law
Church Fathers vs. the "Reformation Pillar" of Faith Alone (Sola Fide) [Including "Revised Protestant Standard" Variant Readings]
Martin Luther on Sanctification and the Absolute Necessity of Good Works as the Proof of Authentic Faith
John Calvin Taught That Good Works Are Part of Every Christian's Life and the Inevitable Manifestation of a True Saving Faith and Justification