A.T. Robertson Commentary Galatians 1:15

"But when it was the good pleasure of God, who separated me, [even] from my mother`s womb, and called me through his grace," — Galatians 1:15 (ASV)

It was the good pleasure of God (ευδοκησεν ο θεος). Paul had no doubt about God's purpose in him (1 Thessalonians 2:8).

Who separated me (ο αφορισας με). Αφοριζω is old word (from απο and ορος) to mark off from a boundary or line. The Pharisees were the separatists who held themselves off from others. Paul conceives himself as a spiritual Pharisee "separated unto the gospel of God" (Romans 1:1, the same word αφωρισμενος). Before his birth God had his plans for him and called him.