Augustine of Hippo, on 1Cor 9:11

By these words the Apostle Paul sufficiently indicates, that it was no usurping unto themselves of aught beyond their due on the part of his fellow-Apostles, that they wrought not bodily, whence they might have the things which to this life are necessary, but as the Lord ordained, should, living by the Gospel, eat bread gratuitously given of them unto whom they were preaching a gratuitous grace. Their charges, namely, they did like soldiers receive, and of the fruit of the vineyard by them planted, they did, as need was, freely gather; and of the milk of the flock which they fed, they drank; and of the threshing-floor on which they threshed, they took their meat. But he speaks more openly in the rest which he subjoins, and altogether removes all causes of doubting. "If we unto you," saith he, "have sown spiritual things, is it a great matter if we shall reap your carnal things?" What are the spiritual things which he sowed, but the word and mystery of the sacrament of the kingdom of heaven? And what the carnal things which he saith he had a right to reap, but these temporal things which are indulged to the life and indigency of the flesh? These however being due to him he declares that he had not sought nor accepted, lest he should cause any impediment to the Gospel of Christ.