Commenting on Galatians 1:15 ("God was pleased to reveal his Son to me"), Adolf Schlatter (1852-1938), professor of NT at Tubingen for 100 straight semesters and author of over 430 works, writes:
The critical turn came through this, that God called Paul. At this point we must think of the experience on the way to Damascus. There God’s call penetrated into his ear, and what called him was God’s grace. When God directed no word of punishment at the persecutor of his church, but rather the call of invitation, which placed him in peace with him and gave him a share in his free gifts, then and only then was grace free. The root of such a call cannot be in those who ascend out of sheer mercy and for whom divine grace bursts through sin.