“For before certain men came from James, he (Peter) used to eat with the Gentiles. But when they arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group.” Galatians 2:12
Peter still wasn’t comfortable eating with Gentiles as it was against Jewish law, despite his experience with Cornelius (Acts 10). Old habits are hard to break. By withdrawing from the Gentiles, Peter was saying, “you’re not good enough, the grace of God isn’t enough to save you.” No wonder Paul “opposed him to his face” (2:11).