(Vers. 17, 18.) Therefore, by the works of the Law, all flesh will not be justified. But if we seek to be justified in Christ, we ourselves have also been found to be sinners, is Christ then a minister of sin? Certainly not! For if the things that I destroyed, I build again, I make myself a transgressor. That flesh, of which it is written, 'All flesh is grass, and all its glory is like the flower of the field' (Isaiah 40:6), will not be justified by the works of the Law. But that flesh of Jesus Christ is justified by faith, of which it is said in the sacrament of the resurrection: All flesh shall see the salvation of God (Luke 3:6). But also according to a lower understanding, not all flesh was justified by the Law, but only those men who were in Palestine. Now, however, all flesh is justified by the faith of Jesus Christ, while his Church is founded throughout the whole world.
Justly, therefore, did he refuse to "build up again (the structure of the law) which he had overthrown." The law, indeed, had to be overthrown, from the moment when John "cried in the wilderness, Prepare ye the ways of the Lord," that valleys and hills and mountains may be filled up and levelled, and the crooked and the rough ways be made straight and smooth -in other words, that the difficulties of the law might be changed into the facilities of the gospel.
How should (the Church) fear to suffer a fraudulent loss of him whom she had already lost on his ereption, and whom, after condemnation, she could not have held? Lastly, to what will it be becoming for a judge to grant indulgence? to that which by a formal pronouncement he has decisively settled, or to that which by an interlocutory sentence he has left in suspense? And, of course, (I am speaking of) that judge who is not wont "to rebuild those things which he has destroyed, lest he be held a transgressor."
Ver. 18. "For if I build up again those things which I destroyed, I prove myself a transgressor."
Observe the Apostle's discernment; his opponents endeavored to show, that he who kept not the Law was a transgressor, but he retorts the argument upon them, and shows that he who did keep the Law was a transgressor, not merely of faith, but of the Law itself. "I build up again the things which I destroyed," that is, the Law; he means as follows: the Law has confessedly ceased, and we have abandoned it, and betaken ourselves to the salvation which comes of faith. But if we make a point of setting it up again, we become by that very act transgressors, striving to keep what God has annulled. Next he shows how it has been annulled.
Note the shrewdness of Paul. For they wanted to show that the one who does not keep the law is a transgressor; but he turns the argument upside down, showing that the one who observes the law is a transgressor not against faith but against the law itself. What he says is as follows: “The law has ceased, as we ourselves agree, in so far as we have left it and taken refuge in the salvation of faith. If we now strive to establish it, we become transgressors by this very fact, as we strive to observe the precepts dissolved by God.”
When they say that he who does not keep the law is a transgressor, he says the exact opposite, calling a transgressor the person who keeps the law. It is like saying: The law has ceased, as we confessed, and so, having abandoned it, we have taken refuge with the salvation which is from faith. If, then, we contest about the application of the law, we become transgressor of the same, inasmuch as we contest about keeping what has been dissolved by God.
[AD 420] Jerome on Galatians 2:17-18