John Chrysostom

Which also he said when writing to the Corinthians. And he says it, to ascribe the whole to God. (1 Cor. ix. 16.) And see him here also unassuming, for he does not say, that I may teach and instruct, but what? "that I might have some fruit." And not fruit, simply, but "some fruit." Again, depreciating his own share therein just as he had said above, "that I may impart some gift." And then to repress them too, as I said also before, he says, "even as among other Gentiles." For, I do not, because you are rich, and have the advantage of …

Homily on Romans 2 ↗

Theophylact of Ohrid

And this is a matter of modesty. I, he says, am not bestowing some favor, but am fulfilling the command of the Master, and you should thank God, for He is the benefactor, while I am "obligated." He said the same thing to the Corinthians as well: "Woe to me if I do not preach the gospel" (1 Cor. 9:16). Therefore I am ready to preach to you as well, even if dangers are before my eyes. Such was his zeal for Christ!

Commentary on Romans ↗

Thomas Aquinas

The other reason is the responsibility of his office: woe to me, if I do not preach the Gospel (1 Cor 9:16). And because he had undertaken the general apostolate of the gentiles, he asserts that he is under obligation to all: although I am free from all men, I have made myself a slave to all (1 Cor 9:19). And for this reason he sets out two diversities. One is along the lines of the diversity of nations, when he says, to Greeks and to barbarians. …

Commentary on Romans ↗