“The life I live in the body, I live by faith in the Son of God, who loved me and gave himself for me”. 
Gal 2:20 NIV
Gratitude is a powerful motivator. When people think carefully and humbly they recall all the things they did nothing to deserve, but received anyway. They recall the home and the upbringing they received. They remember the education they received, the beautiful experiences of childhood, of young love, and (sometimes) of the Christian influences that surrounded them. Maybe they bring to mind kindnesses they were shown, lessons they were taught and examples they lived by.
Paul was powered by gratitude to God for the grace he had received in Christ. Christ had rescued him from the tyranny of the law and from the guilt his failure to fully obey the law had generated. It was all an act of pure grace. He had deserved to be punished. He received - pardon! From struggling unsuccessfully to abide by the (Jewish) law he had come to know the freedom to follow Christ. The wonder of it all never left him. From sin he had been lifted to salvation. From disobedience he had come to discipleship. From murder he had changed to ministry. Grateful to God, he was now grace-filled and lived to share it all with other people. He had seen the cross of Jesus and it had fired him with “a flame of sacred love”. No distance was too far, no people were too far gone to hear and respond to the gospel he so passionately offered.
Be grateful too. Let it inspire you, motivate you and stir you to great deeds for God.

How shall I thank thee for the grace On me and all mankind bestowed?
O that my every breath were praise!
O that my heart were filled with God!
- C. Wesley