James turns to the person who claims faith without works and offers to prove his point. The question means, "Do you want to know?" and it leads straight into his evidence. Barnes says "the reference by this language is to a man who held an opinion that could not be defended." Gill calls him "an empty one, sounding brass, and a tinkling cymbal," someone who boasts of faith but has no works to show it. The question is sharp, but its aim is to teach: James is about to show him from Scripture.

James answers with two examples from the Old Testament. First Abraham: "Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?" (James 2:21). Then Rahab, who "received the messengers, and had sent them out another way" (James 2:25). He concludes, "Ye see then how that by works a man is justified, and not by faith only" (James 2:24), and "faith without works is dead also" (James 2:26).

"Vain" is the Greek kenos, empty. Barnes says it "means properly empty, as opposed to full," like empty hands with nothing in them, and from there it came to mean fruitless, and then false. The last word of the verse differs between Greek texts. The Textus Receptus, behind the KJV, reads nekra, "dead." Modern critical texts read arge, idle or useless, which is why the ESV says "useless." Either way the point stands, since James says "dead" in James 2:17 and James 2:26.

James has just said that even devils believe there is one God (James 2:19). Right belief about God, by itself, is not the faith that saves. This question opens the proof. God's own dealings with Abraham and Rahab show that the faith He counts as righteousness is a faith that acts.

The question also shows how James argues. He does not simply declare the vain man wrong. He offers to show him from Scripture and invites him to look. Matthew Henry's Concise Commentary sums up the teaching of the passage: "Faith is the root, good works are the fruits; and we must see to it that we have both."