Galatians 1:16b (ACCS Ga-Php): Ambrosiaster: When he had faith in the law, not knowing that it was not the time for observance of the law, and was intensely striving to resist the gospel of Christ, he thought that he acted by God’s will. God, seeing that his zeal was good, though he lacked knowledge, chose to summon him into his grace. He knew that this man was suitable to preach his gospel to the Gentiles. For if he was so swift and faithful in so poor a cause through boldness of conscience, not through adulation of anyone, how much more constant would he be in preaching the gift of God through the hope of the promised reward? Epistle to the Galatians 1.15.1.
Galatians 1:18b (ACCS Ga-Php): Jerome: He who had prepared himself for so long a time did not need any long instruction. And, though it seems excessive to some to investigate numbers in Scripture, yet I think it not beside the point to say that the fifteen days that Paul spent with Peter signifies [in late Judaic piety] the fullness of wisdom and the perfection of doctrine, seeing that there are fifteen psalms in a psalter and fifteen steps by which people go up to sing to God. Epistle to the Galatians 1.1.18.
Theodoret: He was called “the brother of the Lord” but was not so by nature. For he was not, as some suppose, the son of Joseph by a previous marriage but the son of Clopas and cousin of the Lord. For his mother was the sister of the Lord’s mother. … He was thought by others to be the Lord’s brother, both because their mothers had the same names and because the families shared one house. And he was so called even by believers, both because of the extreme virtue that he possessed (for he was called “the Just”) and because of the kinship. For the sacred story of the Gospels tells us that the Blessed Virgin had no other son. For seeing her by the cross, the Lord gave her to the most divine John, but he would not have committed her to another if the blessed James, a man possessed of extreme virtue, had been her son. Epistle to the Galatians 1.19.