To whom was Paul called to preach God's Son?

Paul was called to preach God's Son among the Gentiles, also called the heathen. The passage says the revelation was in him so that he might preach Christ among the Gentiles, not so much among the Jews. This commission centered on the Gentile world.

Les explains that in verses 15-16, God, who separated Paul from his mother's womb and called him by grace, revealed His Son in Paul so that he might preach Him among the heathen—the Gentiles. Les emphasizes this was not so much to the Jews but to the Gentiles. He connects this to Saul's Damascus road conversion: though Saul was a total enemy of Christ, Christ saved him by grace rather than removing him. Paul immediately did not confer with flesh and blood, and Les notes there was about a three-year gap before Paul went to Jerusalem, which Galatians fills in.

Galatians 1:15

But when it pleased God, who separated me from my mother’s womb, and called me by his grace,

Galatians 1:16

To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood:

Why did God separate and call Paul from his mother's womb?

God separated Paul from his mother's womb and called him by His grace so God could give him the revelation of Jesus the Son, and so Paul might preach Him among the Gentiles. This gospel came by revelation from the ascended Lord, not from men.