Since a future life is promised to Christians, the one who now lives with God’s assistance lives in the faith of the promised life. For this one contemplates his image, having the pledge of the future life, which was procured for us by Christ’s love in accordance with God’s will. The one who is grateful to Christ is therefore the one who endures in faith toward him. He knows that he has no benefit from anyone but Christ and treats Christ with dishonor if he compares any other to him.
The law could not give remission of sins, nor triumph over the second death nor free from captivity those who were bound because of sin. The reason for Christ’s death was to provide those things that the law could not. He did not die in vain, for his death is the justification of sinners.
Let those, who even now Judaize and adhere to the Law, listen to this, for it applies to them.
"For if righteousness is through the Law, then Christ died for naught."
What can be more heinous than this sin? what more fit to put one to shame than these words? Christ's death is a plain proof of the inability of the Law to justify us; and if it does justify, then is His death superfluous. Yet how could it be reasonable to say that has been done heedlessly and in vain which is so awful, so surpassing human reason, a mystery so ineffable, with which Patriarchs travailed, which Prophets foretold, which Angels gazed on with consternation, which all men confess as the summit of the Divine tenderness? Reflecting how utterly out of place it would be if they should say that so great and high a deed had been done superfluously, (for this is what their conduct came to,) he even uses violent language against them, as we find in the words which follow.
(Verse 21.) I do not reject the grace of God; for if justice is through the Law, then Christ died in vain. He rejects the grace of God, both the one who lives under the Law after the Gospel, and the one who becomes defiled by sins after baptism. But he who can say with the Apostle: His grace in me was not in vain (1 Cor. XV, 10), he also speaks confidently of this: I do not reject the grace of God. What follows, however, is very necessary against those who think that the precepts of the Law must be observed after the faith of Christ. For it must be said to them: If righteousness comes through the law, then Christ died for nothing. Or certainly they should teach how Christ did not die for nothing if works justify. But even though they may be dull, they will not dare to say that Christ died without cause. Therefore, in regard to the participle of the syllogism that is proposed here, that is: If righteousness comes through the law, then Christ died for nothing, we must accept what is consequentially inferred and cannot be denied: but Christ did not die for nothing; and conclude: Therefore, righteousness does not come through the law. So far he has been against Peter, but now he is turning towards the Galatians.
"I do not nullify the grace." As those who still observe to the law are now nullifying the grace through Christ. To nullify is to disbelieve, to despise, to mock. Do you see where he led the argument?
"For if righteousness comes through the law." For if, he says, the law is able to save and to justify, Christ died in vain. For this reason he delivered up himself, as if the law had no power, so that he might save by his death. If indeed the law were saving, the death of the Lord would be unnecessary.
I have been freed through the grace, he says. Therefore, I do not turn back to the Law, nor do I revile the grace as being impotent to vivify. “For if righteousness is through the Law, the Christ has died in vain.” Christ died for us, he says, that he might raise us up, justifying us and removing sin from our midst. But if those who attempt to persuade others to be circumcised say that man is justified in the law, then the death of Christ is made redundant.
After these reasonings, he finally declares: I do not reject the gift of Christ, by which He deemed me worthy, having justified me without works by His death, and I do not resort to the law.
For if, he says, the law is able to save and justify, then Christ died entirely in vain. But He, without a doubt, died in order to save us by His death, which the law is powerless to do. And if the law saves, the death of Christ is superfluous. Do you see where such blasphemy leads?
Then when he says, "I cast not away the grace of God," he draws the principal conclusion. First, he draws the conclusion; secondly, he explains it. He says, therefore: Because I have received from God so great a grace that He delivered Himself, and I live in the faith of the Son of God, "I cast not away the grace of God," i.e., I do not repudiate it or show myself ungrateful: "The grace of God in me hath not been void, but I have labored more abundantly than all they" (1 Cor 15:10). Hence another version has, "I am not ungrateful for the grace of God." "Looking diligently lest any man be wanting to the grace of God" (Heb 12:15), i.e., by showing myself unworthy because of ingratitude.
A form of repudiation and of ingratitude would exist, if I were to say that the Law is necessary in order to be justified. Hence he says, "For if justice be by the law, then Christ died in vain," i.e., if the Law is sufficient, i.e., if the works of the Law suffice to justify a man, Christ died to no purpose and in vain, because He died in order to make us just: "Christ also died once for our sins, the just for the unjust, that he might offer us to God" (1 Pet. 3:18). Now if this could have been done through the Law, the death of Christ would have been superfluous. But He did not die in vain or labor to no purpose, as it is said in Isaiah (49:4); because through Him alone came justifying grace and truth, as it is said in John (1:17). Therefore, if any were just before the passion of Christ, this too was through the faith of Christ to come, in Whom they believed and in Whose faith they were saved.
[AD 384] Ambrosiaster on Galatians 2:21