The following comes from the American Catholic site:
Paul’s entire life can be explained in terms of one experience—his meeting with Jesus on the road to Damascus. In an instant, he saw that all the zeal of his dynamic personality was being wasted, like the strength of a boxer swinging wildly. Perhaps he had never seen Jesus, who was only a few years older. But he had acquired a zealot’s hatred of all Jesus stood for, as he began to harass the Church:
“...entering house after house and dragging out men and women, he handed them over for imprisonment” (Acts 8:3b). Now he himself was “entered,” possessed, all his energy harnessed to one goal—being a slave of Christ in the ministry of reconciliation, an instrument to help others experience the one Savior.
One sentence determined his theology: “I am Jesus, whom you are persecuting” (Acts 9:5b). Jesus was mysteriously identified with people—the loving group of people Saul had been running down like criminals. Jesus, he saw, was the mysterious fulfillment of all he had been blindly pursuing.
From then on, his only work was to “present everyone perfect in Christ. For this I labor and struggle, in accord with the exercise of his power working within me” (Colossians 1:28b-29). “For our gospel did not come to you in word alone, but also in power and in the Holy Spirit and [with] much conviction” (1 Thessalonians 1:5a).
Paul’s life became a tireless proclaiming and living out of the message of the cross: Christians die baptismally to sin and are buried with Christ; they are dead to all that is sinful and unredeemed in the world. They are made into a new creation, already sharing Christ’s victory and someday to rise from the dead like him. Through this risen Christ the Father pours out the Spirit on them, making them completely new.
So Paul’s great message to the world was: You are saved entirely by God, not by anything you can do. Saving faith is the gift of total, free, personal and loving commitment to Christ, a commitment that then bears fruit in more “works” than the Law could ever contemplate.