John Calvin Commentary Galatians 2:12

"For before that certain came from James, he ate with the Gentiles; but when they came, he drew back and separated himself, fearing them that were of the circumcision." — Galatians 2:12 (ASV)

For before that certain persons came. The state of the case is here set forth. For the sake of the Jews, Peter had withdrawn from the Gentiles, intending to drive them from the communion of the Church unless they relinquished the liberty of the Gospel and submitted to the yoke of the Law. If Paul had been silent here, his whole doctrine would have fallen; all the edification achieved through his ministry would have been ruined. It was therefore necessary for him to stand up resolutely and fight with courage.

This shows us how cautiously we should guard against yielding to the opinions of others, lest an excessive desire to please, or an undue fear of causing offense, should turn us away from the right path. If this could happen to Peter, how much more easily could it happen to us, if we are not sufficiently careful!