1 Corinthians 9:19

For though I was free from all men, I brought myself under bondage to all, that I might gain the more.

From this it is clear that it was not merely a matter of justifiable pride that Paul should have insisted on making the gospel free; but it was related to thwarting Satan, as noted under 1 Corinthians 9:15 and for the purpose of procuring a more abundant harvest in the gospel. Moreover, there can be little doubt that Paul's selfless actions actually did result in a mighty increase in the numbers of those accepting the truth. In all ages, there are people of little minds who suppose that every servant of the gospel is more interested in the pecuniary rewards of his work than in the salvation of souls; and, alas, it must be confessed that many times the conduct of preachers themselves supports such allegations.

Under bondage to all ... This has the same ring as Paul's "debtor both to Greeks and barbarians" (Romans 1:14). He accepted for himself the obligation of preaching the gospel "to the whole creation."