12 Here's the situation. Earlier, before certain persons had come from James, Peter regularly ate with the non-Jews. But when that conservative group came from Jerusalem, he cautiously pulled back and put as much distance as he could manage between himself and his non-Jewish friends. That's how fearful he was of the conservative Jewish clique that's been pushing the old system of circumcision.
Matthew Henry's Commentary on Galatians 2:12
Commentary on Galatians 2:11-14
(Read Galatians 2:11-14)
Notwithstanding Peter's character, yet, when Paul saw him acting so as to hurt the truth of the gospel and the peace of the church, he was not afraid to reprove him. When he saw that Peter and the others did not live up to that principle which the gospel taught, and which they professed, namely, That by the death of Christ the partition wall between Jew and Gentile was taken down, and the observance of the law of Moses was no longer in force; as Peter's offence was public, he publicly reproved him. There is a very great 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 think that these ceremonies were necessary.