Read: 1 Corinthians 9:1-23
For when I preach the gospel, I cannot boast, since I am compelled to preach. Woe to me if I do not preach the gospel! If I preach voluntarily, I have a reward; if not voluntarily, I am simply discharging the trust committed to me. What then is my reward? Just this: that in preaching the gospel I may offer it free of charge, and so not make full use of my rights as a preacher of the gospel. 1 Corinthians 9:16-18
What Paul is saying is that he has no sense of pride and achievement because he faithfully preached the gospel without charge. On the contrary, he really has no choice about preaching the gospel. I am compelled to preach. In other words, If I do not preach I am miserable. I have really no choice in this matter. I would much rather preach than experience what I know I am going to experience if I do not: the lash of my conscience, the sense of failure in what God has called me to do. I cannot live with that. Woe to me if I preach not the gospel. He says, If I do it willingly I gain a reward. If I accept this commission from God, and joyfully do what he tells me to do, it is to my great advantage. I enjoy it; but whether I like it or not, I have to do it.
There is nothing wrong with a sense of duty; the feeling that God has given you a job to do, and you have to do it whether you like it or not. Many of us are uneasy with that kind of motivation, but Paul felt it. He said, There is no choice for me in the matter of preaching. Whether I like it or not I have a commission to fulfill, and if I want my life to be worth anything at all, I had better do it.
But that is not why he does it without charge. He tells us the reason in verse 18: What then is my reward? Just this: that in preaching the gospel I may offer it free of charge, and so not make full use of my rights as a preacher of the gospel. He is saying that the thing that motivated him, the thing that drove him to work late hours at night making tents so he would earn a living and would not have to be supported by anybody in the church in Corinth, was the sheer delight it gave him to bless and enrich someone else without taking a penny in return. It was the joy of giving that Paul was experiencing.