Paul confronts Peter because Peter is acting hypocritically. He was not acting in line with the gospel, by treating Gentile Christians as not fully accepted by God through faith. The gospel reminds us that there is no “1st-Class Christian” and “2nd-Class Christian”, but we are all one in Christ – fully accepted by God and equal in our status before him, not because of anything we do or anything we are, but only because of the righteousness of Jesus, credited to us by faith.