Matthew Henry Commentary Galatians 2:11-14

"But when Cephas came to Antioch, I resisted him to the face, because he stood condemned. For before that certain came from James, he ate with the Gentiles; but when they came, he drew back and separated himself, fearing them that were of the circumcision. And the rest of the Jews dissembled likewise with him; insomuch that even Barnabas 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 Cephas before [them] all, If thou, being a Jew, livest as do the Gentiles, and not as do the Jews, how compellest thou the Gentiles to live as do the Jews?" — Galatians 2:11-14 (ASV)

Despite Peter's character, when Paul saw him acting in a way that harmed the truth of the gospel and the peace of the church, he was not afraid to rebuke him. Paul saw that Peter and the others were not living up to the principle that the gospel taught and that they professed—namely, that by the death of Christ the dividing wall between Jew and Gentile had been taken down, and the observance of the Mosaic law was no longer in force; as Peter's offense was public, Paul publicly rebuked him.

There is a very significant difference between the prudence of St. Paul—who bore with, and used for a time the ceremonies of the law as not sinful—and the timid conduct of St. Peter, who, by withdrawing from the Gentiles, led others to believe that these ceremonies were necessary.