Theophylact of Ohrid

He also points out the reason for this rebuke. James was the brother of the Lord, who taught in Jerusalem as their bishop. It was he who sent certain Jews who had already believed but still adhered to the law, and they went to Antioch. Seeing them and fearing not for his own safety, but lest they, being scandalized, should fall away from the faith, Peter began to withdraw from association with the Gentiles. But some, not knowing this reason, began to condemn him.

Commentary on Galatians ↗

Oecumenius

"from James." James, who is called the brother of Christ, served as bishop of Jerusalem. From him therefore those sent forth came who had believed from among the Jews, still observing the law. "he began to draw back and separate himself." From the association with the Gentiles, of course. And the phrase "fearing those" does not mean that he feared they might do him harm, but that he feared so as not to, having been scandalized, they might fall away from the faith.

Commentary on Galatians ↗

Irenaeus

But they themselves, while knowing the same God, continued in the ancient observances; so that even Peter, fearing also lest he might incur their reproof, although formerly eating with the Gentiles, because of the vision, and of the Spirit who had rested upon them, yet, when certain persons came from James, withdrew himself, and did not eat with them. And Paul said that Barnabas likewise did the same thing. …

Against Heresies Book III ↗

Thomas Aquinas

Then when he says, "For, before that some came from James," he manifests what he has said. First, that he said he was to be blamed; Secondly, that he rebuked Peter (v. 14). As to the first he does three things: First, he shows what Peter's opinion was; Secondly, what he did (v. 11); Thirdly, what resulted from it (v. 13). He says therefore, as to the first point, that Peter felt that legalism ought not be observed. …

Commentary on Galatians ↗