7 Of this gospel I was made a minister according to the gift of God's grace, which was given me by the working of his power. 8 To me, though I am the very least of all the saints, this grace was given, to preach to the Gentiles the unsearchable riches of Christ, 9 and to bring to light for everyone what is the plan of the mystery hidden for ages in God who created all things, 10 so that through the church the manifold wisdom of God might now be made known to the rulers and authorities in the heavenly places. 11 This was according to the eternal purpose that he has realized in Christ Jesus our Lord,
Paul calls himself a minister of the gospel, a servant of the good news. This calling is not a vocation he chose for himself. Jesus said, "You have not chosen me, but I have chosen you." He did not earn this right nor does he fulfill his ministry by his own human ability. It is the gift of God's grace and the equipping and empowering of God's Holy Spirit. Those who serve Christ should have the same attitude of humility.
Paul never forgot who he once was nor what great harm he had done to the saints before he met Jesus on the road to Damascus. Because of his past he considered himself less than the least. Humility is a highly prized virtue in Christian service. For one so lowly to bear such a precious treasure is somehow fitting. "We hold this treasure in clay jars." The feeding trough in an animals stall was our Savior's first bed. For thus says the One who is high and lifted up, who inhabits eternity, whose name is Holy: "I dwell in the high and holy place, and also with him who is of a contrite and lowly spirit, to revive the spirit of the lowly, and to revive the heart of the contrite.
To shed rays of light on the mystery. To make plain what has been hidden for ages and generations. This was Paul's great calling. It was on the Damascus road that Jesus Himself had given Paul this mission, "To open their eyes and turn them from darkness to light, and from the power of Satan to God, so that they may receive forgiveness of sins and an inheritance among those who are sanctified by faith in me."
God's plan is to show the innumerable aspects of His wisdom to the entire spirit world by revealing his plan of salvation to His church. Jesus rejection, crucifixion, death and resurrection was not Plan B. He wanted to angels to know that this had been His plan all along.