James 2:20

But wilt thou know - Art thou willing to be instructed in the nature of true saving faith? Then attend to the following examples.

But wilt thou know - Will you have a full demonstration of it; will you have the clearest proof in the case. The apostle evidently felt that the instances to which he was about to refer, those of Abraham and Rahab, were decisive.

O vain man - The reference by this language is to a man who held an opinion that could not be defended. The word "vain" here used (κενε kene) means properly "empty," as opposed to "full" - as empty hands, having nothing in them; then fruitless, or without utility or success; then false, fallacious. The meaning here, properly, would be "empty," in the sense of being void of understanding; and this would be a mild and gentle way of saying of one that he was foolish, or that he maintained an argument that was without sense. James means, doubtless, to represent it as a perfectly plain matter, a matter about which no man of sense could have any reasonable doubt. If we must call a man foolish, as is sometimes necessary, let us use as mild and inoffensive a term as possible - a term which, while it will convey our meaning, will not unnecessarily wound and irritate.

That faith without works is dead - That the faith which does not produce good works is useless in the matter of salvation. He does not mean to say that it would produce no effect, for in the case of the demons it did produce trembling and alarm; but that it would be valueless in the matter of salvation. The faith of Abraham and of Rahab was entirely different from this.

2:20 But art than willing to know - Indeed thou art not: thou wouldest fain be ignorant of it. O empty man - Empty of all goodness. That the faith which is without works is dead - And so is not properly faith, as a dead carcase is not a man.