17 But would you know that the Apostles knew and preached the name of Christ, or rather had Christ Himself within them? Paul says to his hearers, Or seek a proof of Christ that speaks in me? Paul proclaims Christ, saying, For we preach not ourselves, but Christ Jesus as Lord, and ourselves your servants for Jesus' sake. Who then is this? The former persecutor. O mighty wonder! The former persecutor himself preaches Christ. But wherefore? Was he bribed? Nay there was none to use this mode of persuasion. But was it that he saw Him present on earth, and was abashed? He had already been taken up into heaven. He went forth to persecute, and after three days the persecutor is a preacher in Damascus. By what power? Others call friends as witnesses for friends but I have presented to you as a witness the former enemy: and do you still doubt? The testimony of Peter and John, though weighty, was yet of a kind open to suspicion: for they were His friends. But of one who was formerly his enemy, and afterwards dies for His sake, who can any longer doubt the truth?
Source: Catechetical Lectures (New Advent)