The following is an excerpt from The MacArthur New Testament Commentary on Galatians 2.

But when Cephas came to Antioch, I opposed him to his face, because he stood condemned. (2:11)Because the Judaizers had told believers in the Galatian churches that Paul was not a true apostle, the incident mentioned in this verse is especially significant. Paul not only was equal to the other apostles but had on this occasion even reprimanded Peter (Cephas), the one who was recognizably the leading apostle among the Twelve. Both Peter and Paul had experienced salvation by grace through faith, both were directly chosen by the resurrected Jesus Christ to be apostles, and both had been mightily used by the Holy Spirit in establishing and teaching the church. The book of Acts can be divided between the early church ministry that centered on Peter (1–12) and that which centered on Paul (13–28). But in Antioch these two men of God came into head-on collision.

Opposed is from anistemi, which carries the meaning of hindering or forbidding, and was usually applied to defensive measures. By his withdrawal from the Gentiles, Peter had, in effect, joined the Judaizers in belittling Paul’s inspired teaching, especially the doctrine of salvation by God’s grace alone working through man’s faith alone. Peter knew better, and Paul opposed him to his face, because he stood condemned.

Peter was not condemned in the sense of losing his salvation but in the sense of being guilty of sin by taking a position he knew was wrong. He no doubt also stood condemned as a sinner in the eyes of the Gentile believers in Antioch, who, because they were well-grounded in the gospel of grace, were perplexed and deeply hurt by his ostracism of them.

Before Peter’s compromise with the Judaizers could do serious damage in the Antioch church, God used Paul to nip the error in the bud. In so doing He also provided Paul with perhaps his most convincing proof of apostolic authority. God has a purpose even in the worst of circumstances, and what could have been a tragedy He used for His glory and for the strengthening of His church.