Charles Ellicott 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; you do well.—Better thus, You believe that God is One; you do well. He is the formal object of faith derived from knowledge, whether by sense, intuition, or demonstration; you are theologically correct, and may even declare your internal faith by external confession—well, indeed.

The devils also believe, and tremble.—They shudder in the belief which only assures them of their utter misery; literally, their hair stands on end with terror of the God they acknowledge. Assent, opinion, knowledge—all are thus shared by demons of the pit; do not call this possession you share with them by the holier name of Faith. “I believe in God,” “I believe in one God”—such is the voice of the Christian; and this is said in the full sense “only by those who love God, and who are not only Christians in name, but in deed and in life.”