What does James 2:20 mean?

In prior verses, James has attempted to make a clear distinction between a so-called-"faith" and a truly saving faith. False faith is one which results in no good works. The works do not save us, and there is no sense in which James requires works for salvation. But, he points out that even demons have a sort of "faith," by believing that God exists. Faith which saves is faith which results in action.

James is not arguing that faith doesn't matter. Nor is he denying that through faith alone we are saved by God's grace (Ephesians 2:8–9). He is simply noting that the "faith which saves" is something more than an opinion. Instead, James is arguing for exactly what Paul wrote in Ephesians 2:10: That the God who saved us by His grace through faith—and not in any sense through works—planned all along for His believing children to do good works, "that we should walk in them." That's what true believers do.

With this short verse, James prepares to make his final arguments that genuine faith always results in good works. He asserts in rather blunt terms that those who disagree are foolish. He asks his hypothetical, foolish "someone," the source of the challenge of verse 18, if he wants to be shown or see evidence that faith without works is a useless, dead faith.