James 2:19

Thou believest that there is one God
These words are a continuation of the address of the man that has works, to him that boasts of his faith without them, observing to him, that one, and a main article of his faith, is, that there is one God; which is to be understood in the Christian sense, since both the person speaking, and the person spoken to, were such as professed themselves Christians; so that to believe there is one God, is not merely to give into this article, in opposition to the polytheism of the Gentiles, or barely to confess the God of Israel, as believed on by the Jews, but to believe that there are three persons, Father, Son, and Spirit, and that these three are the one God; wherefore this article of faith includes everything relating to God; as to God the Father, his being and perfections, so to Christ, as God, and the Son of God, and the Messiah and to the Holy Spirit; and to believe all this is right:

thou doest well;
for that there is but one God, is to be proved by the light of nature, and from the works of creation and providence, and has been owned by the wisest of the Heathens themselves; and is established, by divine revelation, in the books both of the Old and of the New Testament; what has been received by the Jews, and is well known by Christians, to whom it is set in the clearest light, and who are assured of the truth of it: but then

the devils also believe;
the Arabic version reads, "the devils likewise so believe"; they believe the same truth; they know and believe there is but one God, and not many; and they know that the God of Israel is he; and that the Father, Son, and Spirit, are the one God; they know and believe him to be the most high God, whose servants the ministers of the Gospel are; and they know and believe that Jesus is the Holy One of God, the Son of God, and the Messiah, ( Acts 16:17 ) ( 19:15 ) ( Luke 4:34 Luke 4:41 ) .

And tremble;
at the wrath of God, which they now feel, and at the thought of future torments, which they expect, ( Mark 5:7 ) ( Matthew 8:29 ) and which is more than some men do; and yet these shall not be saved, their damnation is certain and inevitable, ( 2 Peter 2:4 ) ( Jude 1:6 ) wherefore it follows, that a bare historical faith will not profit, and cannot save any; a man may have all faith of this kind, and be damned; and therefore it is not to be boasted of, nor trusted to.