When Cephas came to Antioch, I opposed him to his face, because he stood self-condemned; for until certain people came from James, he used to eat with the Gentiles. But after they came, he drew back and kept himself separate for fear of the circumcision faction. And the other Jews joined him in this hypocrisy, so that even Barnabas was led astray by their hypocrisy. But when I saw that they were not acting consistently with the truth of the gospel, I said to Cephas before them all, "If you, though a Jew, live like a Gentile and not like a Jew, how can you compel the Gentiles to live like Jews?" (Galatians 2:11-14, NRSV)
When Peter was come to Antioch, I withstood him to the face, because he was to be blamed. For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. And the other Jews dissembled likewise with him; insomuch that Barnabas also was carried away with their dissimulation. But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Peter before them all, If thou, being a Jew, livest after the manner of Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews? (Galatians 2:11-14, KJV)
Paul really gets sore when he sees anyone turning from the truth. Peter decides not to upset the other Christians visiting from Jerusalem, and Paul calls him out on it. After all, if there's only one truth, then acting any other way is flying in the face of what's real and good.