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.
Ver. 21. "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.
How could an act so great, so awesome and surpassing human reason be to no purpose? How could a mystery so ineffable, for which the prophets yearned in travail, the patriarchs foresaw and the angels were astonished to behold, acknowledged by all as the crown of God’s loving care—how could one say that this was vain and futile? Considering therefore how exceedingly absurd it would be for them to say a deed of such significance and magnitude had been superfluous … Paul adopts an indignant tone toward them saying, “O foolish Galatians.”
(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 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.
[AD 384] Ambrosiaster on Galatians 2:21