God, who set me apart from my mother’s womb: God chose Paul for special work even before he was born. This is like how God chose other leaders (see Jeremiah 1:5; Psalm 139:13–16; Isaiah 49:1–6; Romans 9:10–13).
called me by His grace: For Paul's conversion story (how he became a follower of Jesus), see Acts 9:1–19; 22:1–21; 26:9–23.
Paul was called to share the good news about Jesus with non-Jews (Galatians 2:7). He even taught other apostles what this meant (see Galatians 2:11–21).
to reveal His Son in me: This might refer to Paul's meeting with Christ (Acts 9:1–19; see also 1 Corinthians 9:1; 15:8). In me might also mean an inner revelation (see Galatians 2:20; 4:6–7) and suggests that Paul's changed heart showed Christ to others.