Ephesians 3: Mystery revealed.

Paul was chosen by the Lord to reveal a great mystery:- gentiles are also chosen to serve the Messiah.

4-6 As you read over what I have written to you, you’ll be able to see for yourselves into the mystery of Christ. None of our ancestors understood this. Only in our time has it been made clear by God’s Spirit through his holy apostles and prophets of this new order. The mystery is that people who have never heard of God and those who have heard of him all their lives (what I’ve been calling outsiders and insiders) stand on the same ground before God. They get the same offer, same help, same promises in Christ Jesus. The Message is accessible and welcoming to everyone, across the board.

Paul was a pharisee, proud about his Jewish heritage. As a pharisee, he saw non Jews as people outside the covenant, lost in their idolatry and somehow despised by him. However, by grace, the Lord Jesus chose Paul to reveal a mystery that could never foresee. Jesus filled Paul with love, and Jesus enabled Paul to love the gentiles, to the point of being persecuted by other Jews for opening the covenant to non Jews.
It is like a religious person, grown in a church, who despises Muslims, and one day, by grace, he understands that he is loved by Jesus and want to spread that love to Muslims.