Jamieson, Fausset and Brown on James 2:19

19. Thou--emphatic. Thou self-deceiving claimant to faith without works.

that there is one God--rather, "that God is one": God's existence, however, is also asserted. The fundamental article of the creed of Jews and Christians alike, and the point of faith on which especially the former boasted themselves, as distinguishing them from the Gentiles, and hence adduced by James here.

thou doest well--so far good. But unless thy faith goes farther than an assent to this truth, "the evil spirits (literally, 'demons': 'devil' is the term restricted to Satan, their head) believe" so far in common with thee, "and (so far from being saved by such a faith) shudder (so the Greek)," Mt 8:29; Lu 4:34; 2Pe 2:4; Jude 6; Re 20:10. Their faith only adds to their torment at the thought of having to meet Him who is to consign them to their just doom: so thine (He 10:26, 27, it is not the faith of love, but of fear, that hath torment, 1Jo 4:18).