Evidently Peter had gone back from the clear revelation of Act 10:1-48, and from his former practice as stated in Gal 2:12. The fear of the conservative party of the mother Church had brought him into a snare. His example had a very unfortunate effect upon the rest of the Hebrew Christians, who took their lead from him. But Paul’s remonstrance probably brought Peter back to his former and happier practice.
Paul goes on to show that the death of Christ has taken us altogether out of the realm of the ancient Law, with its restrictions and distinctions between clean and unclean, Jew and Gentile, Gal 2:15-19. If the conservative view was right, and it was wrong to eat with the Gentiles, then all that Christ had done and taught was in vain. Indeed, he had become a minister to sin, Gal 2:17, because he had taught his people to associate with Gentiles. But such a suggestion was, of course, unthinkable, and therefore Peter was wrong in withdrawing from Gentile fellowship.
Then the Apostle breaks out into the memorable confession of the power of the Cross in his own life, Gal 2:20-21. It stood between him and the past. His self-life was nailed there, and this new life was no longer derived from vain efforts to keep the Law, but from the indwelling and uprising of the life of Jesus-the perennial spring of Joh 4:14.