But wilt thou know. We must understand the state of the question, for the dispute here is not respecting the cause of justification, but only what avails a profession of faith without works, and what opinion we are to form of it. Absurdly then do they act who strive to prove by this passage that man is justified by works, because James meant no such thing, for the proofs which he subjoins refer to this declaration, that no faith, or only a dead faith, is without works. No one will ever understand what is said, nor judge wisely of words, except he who keeps in view the design of the writer.

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.

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

(11) But wilt thou know, O vain man, that faith without works is dead?
(11) The third reason from the example of Abraham, who no doubt had a true faith: but he in offering his son, showed himself to have that faith which was not without works, and therefore he received a true testimony when it was laid, that faith was imputed to him for righteousness.

But wilt thou know, O vain man,.... These are the words of the apostle reassuming the argument, that faith without works is dead, useless, and unprofitable; and the man that boasts of his faith, and has no works to show it, he calls a "vain man", an empty one, sounding brass, and a tinkling cymbal; empty vessels make the greatest sound; such are proud boasters, vainly puffed up by their fleshly mind; but are empty of the true knowledge of God, and of the faith of Christ, and of the grace of the Spirit: the Syriac version renders it, "O weak", or "feeble man", as he must needs be, whose faith is dead, and boasts of such a lifeless thing; and the Ethiopic version renders it, "O foolish man", for such an one betrays his ignorance in spiritual things, whatever conceit he has of his knowledge and understanding: the character seems levelled against the Gnostics, who were swelled with a vain opinion of their knowledge, to whom the apostle addresses himself thus. The phrase, "vain man", is a proper interpretation of the word "Raca", or Reka, used in Matthew 5:22; see Gill on Matthew 5:22, which though not to be said to a man in an angry way, yet may be applied to men of such a character as here described; who were empty of solid good, and yet boasted of their knowledge. "Wilt thou know?" dost thou require proofs,
that faith, without works, is dead? as in James 2:17 and that true faith has always works accompanying it, and is shown and known by it? then take the following instances.

wilt thou know--"Vain" men are not willing to know, since they have no wish to "do" the will of God. James beseeches such a one to lay aside his perverse unwillingness to know what is palpable to all who are willing to do.
vain--who deceivest thyself with a delusive hope, resting on an unreal faith.
without works--The Greek, implies separate from the works [ALFORD] which ought to flow from it if it were real.
is dead--Some of the best manuscripts read, "is idle," that is, unavailing to effect what you hope, namely, to save you.

But wilt thou know, etc. "I will show by proof that the faith that justifies produces works by the case of Abraham, the great father of the faithful."
Faith without works is dead. It has no power to make alive unless it has power over the life.
Was not Abraham our father. The father of the Jewish race; also the father of all his children by faith. See Galatians 3:7-9.
Justified by works. By a faith which showed itself in works. This is the argument of James, that faith is of no avail unless accompanied by works. Of this all Abraham's life was a demonstration. He was told when in Mesopotamia to go forth into the land God would show him, and "he went forth, not knowing whither;" an act of faith (Hebrews 11:8); "By faith he sojourned in the land of promise" (Hebrews 11:9). Each of these acts of faith secured Divine approval, but the supremest trial was when he offered Isaac. See notes on Hebrews 11:17. This victory of faith was followed by crowning approval and glorious promises.
Seest thou how faith wrought. Abraham's faith was a working faith.
By works was faith made perfect. The element of works is essential to make it complete. Without works it is an imperfect, "a dead faith." It must have energy to avail. Abraham's faith "wrought."
And the scripture was fulfilled. The scripture quoted is Genesis 15:6. The case of the offering of Isaac was a complete outward demonstration that Abraham believed God, as the Scripture said. "He was called the friend of God." Honored as no other mortal had been. See Isaiah 41:8, and 2-Chronicles 20:7. The Mohammedans still call Abraham El Khalil, "The Friend."
Ye see then, etc. The case of Abraham proves that the justifying faith is a working faith.

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.