O foolish man, do you want evidence that faith without deeds is worthless?
20 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.
In this latter part of the chapter, the apostle shows the error of those who rested in a bare profession of the Christian faith, as if that would save them, while the temper of their minds and the tenour of their lives were altogether disagreeable to that holy religion which they professed.
Commenting on James 2:14-26
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...