Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
Thou - emphatic. Thou self-deceiving claimant to faith without works.
That there is one-God, х ho (G3588) Theos (G2316) estin (G2076) heis (G1520)] - 'that God is one:' God's existence is also asserted. The fundamental article of Jews and Christians alike; the point of faith on which especially the former boasted themselves, as distinguishing them the Gentiles; hence, adduced by James here.
Thou doest well. But unless thy faith goes further than an assent to this truth, 'the demon ("devil" is restricted to Satan, their head) believe' in common with thee, 'and (so far from being saved such a faith) shudder' х frissousin (G5425)] (Matthew 8:29; Luke 4:34; 2 Peter 2:4; Jude 1:6; Revelation 20:10). Their faith only adds to their torment at having to meet Him who is to consign them to their just doom: so thine (Hebrews 10:26-27), not the faith of love, but fear, that hath torment, (1 John 4:18).