But when Cephas came to Antioch, I opposed him to his face, because he stood condemned. For before certain men came from James, he was eating with the Gentiles; but when they came he drew back and separated himself, fearing the circumcision party. And the rest of the Jews acted hypocritically along with him, so that even Barnabas was led astray by their hypocrisy.So what we have is Peter, a Jew, who ate with Gentiles, was being consistent with the fellowship all believers in Christ were supposed to have. Then there's the circumcision group who held that believers had to follow the laws laid down by Moses as well as have faith in Christ in order to be saved. This meant meant believers had to have a form of "works righteousness" (ie circumcision). Long story short, when the circumcision party arrived, Peter crumbled out of fear of what they might do. Rather than remaining where he was and so testify to the unity of all believers, he separated himself from the Gentiles instead, thus denying this unity. Such was Peter's position and influence that he also led a number of other believers in his error - including the apostle Barnabas!
Paul states "Cephas stood condemned." What this tells me is that Paul's reason for acting wasn't based on anything about Paul but out of concern for Peter and the people who were with him. Paul didn't use his position, authority, or any other "worldly" reason to abuse and condemn Peter for his action, nor did he whisper behind his back, hide the issue, talk to other people, etc. Paul discharged the responsibility that God had given him - he confronted Peter and everyone else involved in that mess face to face.
An important point to get here is that while Peter was the main offender due to his position of leadership, he wasn't the sole offender. The other Jews who were with Peter could've stayed where they were, but they didn't - they chose to join Peter in his hypocrisy and separate from the Gentiles. This was an offence committed by a group of people, not just one person. As such, it was right and appropriate for Paul to confront Peter in front of these other people, because they had sinned as well. Paul was addressing a number of people who had sinned, but Peter was probably focused out because he as an apostle should've known - and acted - better. Those who God places in positions of authority and responsibility will be judged more strictly than others (James 3:1).
Continuing on -
"But when I saw that their conduct was not in step with the truth of the gospel, I said to Cephas before them all, “If you, though a Jew, live like a Gentile and not like a Jew, how can you force the Gentiles to live like Jews?See how Paul opens his confrontation - not with an accusation or condemnation, but with a question. A question that had what I would surmise to be a self-evident answer to Peter and the rest of the people with him.
"We who are Jews by birth and not ‘Gentile sinners’ know that a man is not justified by observing the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by observing the law, because by observing the law no one will be justified. If, while we seek to be justified in Christ, it becomes evident that we ourselves are sinners, does that mean that Christ promotes sin? Absolutely not! If I rebuild what I destroyed, I prove that I am a lawbreaker. For through the law I died to the law so that I might live for God. I have been crucified with Christ and I no longer live, but Christ lives in me. The life I live in the body, I live by faith in the Son of God, who loved me and gave himself for me. I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing!”And here's the whole crux of the situation - the circumcision party believed other works were required in addition to Christ's work of justification in order to be saved. Were this true, then Christ's death would be for nothing.
Peter, by separating from the Gentiles who didn't follow the Jewish traditions, implicitly agreed with the circumcision party. His example then led a number of other believers into hypocrisy, which is one of the most repeatedly condemned sins in Scripture - and why Peter "stood condemned."
Looking at this from another perspective - namely how to address a person that's in error - we see Paul addressing the issue, not the person. He starts out with a rhetorical question that Peter certainly would be able to answer, and then proceeds to gently but factually show how Peter's actions was actually a denial of Christ's death and resurrection.
This is something for all of us to consider the next time we find ourselves dealing with an erring sibling in Christ.