Charles Ellicott Commentary 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." — 1 Corinthians 9:19 (ASV)

For.—The question is here answered. His reward was to gain the greater number of converts—Jews (1 Corinthians 9:20), Gentiles (1 Corinthians 9:21), weak ones (1 Corinthians 9:22). The only reward he sought or looked for in adopting that course of conduct, for which they taunted him with selfishness, was, after all, their good.

The word "For," introducing the answer, would seem to imply that the reward must be a greater one. For though an Apostle, I became a slave of all that I might gain the greater number. The words "greater number" probably include two ideas: namely, a greater number than he could have gained had he used his rights as an Apostle, and also a greater number of converts than was gained by any other Apostle.