Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God...
Romans 1:1 ESV

Paul was saying that he was a personal possession and emissary of Yeshua due to one of three reasons: 1) He was in debt to Yeshua beyond hope of paying it back, 2) He sold himself to Yeshua to pay a debt to someone else, or 3) He was a slave for one of the two prior reasons, but voluntarily submitted to his master for life out of love.