By the will of God (Colossians 1:1)

Paul's apostleship is not a role he chose for himself. God chose him in a dramatic and forceful way. According to Acts 9:1-22, Paul was determined to destroy the church but God stopped him on the way and imposed His will upon him (vs. 3-6, 15). Paul was an apostle and preacher of the gospel because he had been given a stewardship by God. He was compelled to this ministry even against his will (1 Cor. 9:16, 17).