Before certain men came from James he [Peter], used to eat with the Gentiles. But when they arrived, he began to draw back. Galatians 2:12 (NIV)
If you read the New Testament carefully, you will discover that there was a great struggle going on about the mission of the church. The apostles in Jerusalem were focused on evangelism among the Jews, and Paul had been commissioned by Christ to go to the Gentiles.
The word gentile means everybody who is not Jewish, so let’s substitute the phrase the whole world for the Gentiles, to understand Paul’s description: “Those men [the other apostles] added nothing to my message. On the contrary, they saw that I had been entrusted with the task of preaching the gospel to the [whole world], just as Peter had been to the Jews… James, Peter, and John, those reputed to be pillars, gave me and Barnabas the right hand of fellowship when they recognized the grace given to me. They agreed that we should go to [the whole world] and they to the Jews” (2:6-7, 9).
That was the agreement. They shook hands on it and off they went to pursue their different ministries, and that went fine, until Peter came to Antioch (2:11). Then something went terribly wrong. “Before certain men came from James, he [Peter] used to eat with the Gentiles. But when they arrived, he began to draw back” (2:12).
Peter pulled back from the Gentiles because he was taking the heat from a powerful group of people who wanted to impose the distinctives of their Jewish heritage on other believers, and for this Paul ended up rebuking him.
Have you grasped that the gospel is for the world? Or have you been operating like the Christian life is about gathering a few people like yourself and enjoying the warmth of your fellowship together?