Matthew Henry’s Commentary on the Whole Bible
1. Peter's fault. When he came among the Gentile churches, he complied with them, and did eat with them, though they were not circumcised, agreeably to the instructions which were given in particular to him (Acts 10), when he was warned by the heavenly vision to call nothing common or unclean. But, when there came some Jewish Christians from Jerusalem, he grew more shy of the Gentiles, only to humour those of the circumcision and for fear of giving them offence, which doubtless was to the great grief and discouragement of the Gentile churches. Then he withdrew, and separated himself. His fault herein had a bad influence upon others, for the other Jews also dissembled with him; though before they might be better disposed, yet now, from his example, they took on them to scruple eating with the Gentiles, and pretended they could not in conscience do it, because they were not circumcised. And (would you think it?) Barnabas himself, one of the apostles of the Gentiles, and one who had been instrumental in planting and watering the churches of the Gentiles, was carried away with their dissimulation. Here note, (1.) The weakness and inconstancy of the best of men, when left to themselves, and how apt they are to falter in their duty to God, out of an undue regard to the pleasing of men. And, (2.) The great force of bad examples, especially the examples of great men and good men, such as are in reputation for wisdom and honour.
2. The rebuke which Paul gave him for his fault. Notwithstanding Peter's character, yet, when he observes him thus behaving himself to the great prejudice both of the truth of the gospel and the peace of the church, he is not afraid to reprove him for it. Paul adhered resolutely to his principles, when others faltered in theirs; he was as good a Jew as any of them (for he was a Hebrew of the Hebrews), but he would magnify his office as the apostle of the Gentiles, and therefore would not see them discouraged and trampled upon. When he saw that they walked not uprightly, according to the truth of the gospel - that they did not live up to that principle which the gospel taught, and which they had professed to own and embrace, namely, that by the death of Christ the partition-wall between Jew and Gentile was taken down, and the observance of the law of Moses was no longer in force - when he observed this, as Peter's offence was public, so he publicly reproved him for it: He said unto him before them all, If thou, being a Jew, livest after the manner of the Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews?