"But when he who had set me part before I was born,1 and who called me by his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles (Gal 1:15-16a ESV)."
Paul is arguing against Judaisms in the church at Galatia. Judaizers were 'Christians' who were forcing Gentiles to adopt or practice Old Testament Jewish laws as an essential part of the gospel (called works of law). Paul declared this to be a different gospel (1:6) and a contrary gospel (1:8). As part of his argument that the genuine gospel is purely of grace he makes three claims about his salvation, that God set him apart literally in his mothers womb, that God called him by grace, and that God revealed Jesus to him. Notice that God is the subject and the one doing the in all three of these actions. God set him apart prior to birth, hence not any work on his part. God also called him by grace, and last God found great pleasure in revealing his Son to Paul. This last statement is reminiscent of 2 Cor 4:6, where Paul describes God as having shone in our hearts to give us the light of the knowledge of the glory of God in the face of Christ. He compared this to the act of creation where God said 'let there be light.' In the same way, God acted upon our minds so that in a moment, for the first time, we saw the glory of God in Christ. His point in the three claims is that salvation and even sanctification is work of God and not the work of man.