John Calvin Commentary James 2:19

"Thou believest that God is one; thou doest well: the demons also believe, and shudder." — James 2:19 (ASV)

You believe that there is one God. From this one sentence it appears evident that the whole dispute is not about faith, but about the common knowledge of God. This knowledge can no more connect man with God than the sight of the sun can carry him up to heaven. However, it is certain that by faith we come near to God.

Besides, it would be ridiculous if anyone were to say that the devils have faith; and James prefers them in this respect to hypocrites. The devil trembles, he says, at the mention of God’s name, because when he acknowledges his own judge, he is filled with the fear of him. Therefore, the one who despises an acknowledged God is much worse.

You do well, is stated to diminish its importance, as if he had said, “It is, indeed! a great thing to sink down below the devils.”