John Calvin on James 2:15-16

15 If a brother, or, For if a brother. He takes an example from what was connected with his subject; for he had been exhorting them to exercise the duties of love. If any one, on the contrary, boasted that he was satisfied with faith without works, he compares this shadowy faith to the saying of one who bids a famished man to be filled without supplying him with the food of which he is destitute. As, then, he who sends away a poor man with words, and offers him no help, treats him with mockery, so they who devise for themselves faith without works, and without any of the duties of religion, trifle with God. This is adduced as an illustration: as the saying of a man to the naked, “Be ye clothed,” when he does nothing, effects no good, is wholly useless, so is that faith that produces no works; it being as it were dead, it cannot save.