John Chrysostom — on Gal 1:1 (Homily on Galatians 1)

“The not being "from men" has reference to all alike for the Gospel's root and origin is divine, but the not being "through man" is peculiar to the Apostles; for He called them not by men's agency, but by His own. But why does he not speak of his vocation rather than his apostolate, and say, "Paul" called "not by man?" Because here lay the whole question; for they said that the office of a teacher had been committed to him by men, namely by the Apostles, whom therefore it behooved him to obey. But that it was not entrusted to him by men, Luke declares in the words, "As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul."”