For if I preach the gospel, that gives me no ground for boasting. For necessity is laid upon me. Woe to me if I do not preach the gospel! For if I do this of my own will, I have a reward, but if not of my own will, I am still entrusted with a stewardship. What then is my reward? That in my preaching I may present the gospel free of charge, so as not to make full use of my right in the gospel.
In chapter nine of this letter to the church at Corinth, God has lead Paul to write about his own authority and standing in and toward this church. In earlier chapters he has exposed the division in the church that some had loyalty for one and some loyalty for another and some who had loyalty for Paul. He is correcting this devision by instructing them about his conduct in their midst. Whereas some have taken advantage of them by collecting funds from them for their ministry, he has not. He is not saying that ministers of the gospel should not be paid by the gospel. In fact, just the opposite:
1 Corinthians 9:14 (ESV)
In the same way, the Lord commanded that those who proclaim the gospel should get their living by the gospel.
But this is where he separates himself from the others in Corinth, also proclaiming the gospel. He has not afforded himself with that right. Instead, as he states in the above few verses, he has a compulsion to preach the gospel. He must preach it. It is a necessity that is laid upon him. Oh, would that be true for us all! It is not about leveraging the gospel for gain. It is about gaining the gospel more leverage in the hearts of those you meet. Paul had to preach the gospel. He could not, not. He believes teaching and preaching the gospel is a gift he is to steward. As the disciples were commissioned to go into the world to preach the gospel, so, too, Paul. This is, according to his argument, the proof of his apostleship and the reason the church at Corinth should listen to him. Not because of who he was, but because of his message. So, we too, ought to preach the gospel. We should believe, like Paul, we are compelled to do so.