Adam Clarke on 1 Corinthians 9:16

For though I preach the Gospel - I have cause of glorying that I preach the Gospel free of all charges to you; but I cannot glory in being a preacher of the Gospel, because I am not such either by my own skill or power. I have received both the office, and the grace by which I execute the office, from God. I have not only his authority to preach, but that authority obliges me to preach; and if I did not, I should endanger my salvation: yea, wo is unto me, if I preach not the Gospel. As every genuine preacher receives his commission from God alone, it is God alone who can take it away. Wo to that man who runs when God has not sent him; and wo to him who refuses to run, or who ceases to run, when God has sent him.