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 wordaqyr, "Raca", or Reka, used in Mt 5:22,See Gill on "Mt 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 Jas 2:17 and that true faith has always works accompanying it, and is shown and known by it?