Galatians 1:1

Paul, an apostle, not of men. That is, because the other Apostles were sent by Christ while still mortal, Paul by Christ when wholly deified, and therefore in every way immortal. So says S. Augustine. But the simpler explanation is to take not of men to mean, not of mere men, but of Christ, man and God.

There is a fourfold mission, says S. Jerome. Some are sent by God alone, as Paul; some by God through man's instrumentality, as Joshua was through Moses; some by man alone, as those who are promoted by their friends to be abbot, dean, or bishop; some by themselves, as heretics. The preposition "of" (ab) therefore, used here, denotes the principal cause, while "by" (per) denotes the instrumental; for the meaning is that he was not called by man, nor by God by means of man, but immediately by God Himself.