Albert Barnes Commentary 1 Corinthians 9:17

"For if I do this of mine own will, I have a reward: but if not of mine own will, I have a stewardship intrusted to me." — 1 Corinthians 9:17 (ASV)

For if I do this thing willingly. If I preach so as to show that my heart is in it, that I am not compelled. If I pursue such a course as to show that I prefer it to all other employments. If Paul took compensation for his services, he could not properly do this; if he did not, he showed that his heart was in it and that he preferred the work to all others. Even though he had been in a way compelled to engage in that work, yet he so acted in the work as to show that it had his wholehearted preference. This was done by his submitting to voluntary self-denials and sacrifices to spread the Savior's name.

I have a reward. I will receive the approval of my Lord and will obtain the reward in the world to come which is promised to those who engage wholeheartedly, laboriously, and successfully in turning sinners to God, Proverbs 11:30; Daniel 12:3; Matthew 13:43; Matthew 25:21–23; James 5:20.

But if against my will. akwn. If under a necessity (1 Corinthians 9:16); if by the command of another—(Grotius); if I do it by the fear of punishment, or by any strong necessity which is laid on me.

A dispensation of the gospel is committed unto me. I am entrusted with (pepisteumai) this dispensation, office, economy (oikonomian) of the gospel. It has been laid upon me; I have been called to it; I must engage in this Work; and if I do it from mere compulsion, or in such a way that my will does not acquiesce in it and concur with it, I will have no distinguished reward.

The work must be done; I must preach the gospel; and it is fitting for me to do it in such a way as to show that my heart and will entirely concur, that it is not a matter of compulsion, but of choice. This he proposed to do by so denying himself, and so foregoing comforts which he might lawfully enjoy, and so subjecting himself to perils and toils in preaching the gospel, as to show that his heart was in the work, and that he truly loved it.