Thou believest that there is one God. From this one sentence it appears evident that the whole dispute is not about faith, but of the common knowledge of God, which can no more connect man with God, than the sight of the sun carry him up to heaven; but it is certain that by faith we come nigh to God. Besides, it would be ridiculous were any one 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. He then who despises an acknowledged God is much worse. Thou doest well, is put down for the purpose of extenuating, as though he had said, "It is, forsooth! a great thing to sink down below the devils." [1]

Footnotes

1 - The design of alluding to the faith of devils seems to have been this, to shew that though a good man may believe and tremble, yet if he does not obey God and do good works, he has no true evidence of faith. Obedient faith is that which saves, and not merely that which makes us tremble. The connection with the preceding verse seems to be as follows, -- In the former verse the boaster of mere faith is challenged to prove that his faith is right and therefore saving; the challenger would prove by his works. Then, in this verse, a test is applied -- the very first article of faith is mentioned: "Be it that you believe this, yet this faith will not save you: the devils have this faith, and instead of being saved they tremble.

Thou believest that there is one God - One of the great and cardinal doctrines of religion is here selected as an illustration of all. The design of the apostle seems to have been to select one of the doctrines of religion, the belief of which would - if mere belief in any doctrine could - save the soul; and to show that even this might be held as an article of faith by those who could be supposed by no one to have any claim to the name of Christian. He selects, therefore, the great fundamental doctrine of all religion, - the doctrine of the existence of one Supreme Being, - and shows that if even this were held in such a way as it might be, and as it was held by devils, it could not save men. The apostle here is not to be supposed to be addressing such an one as Paul, who held to the doctrine that we are justified by faith; nor is he to be supposed to be combating the doctrine of Paul, as some have maintained, (see the Introduction); but he is to be regarded as addressing one who held, in the broadest and most unqualified sense, that provided there was faith, a man would be saved. To this he replies, that even the devils might have faith of a certain sort, and faith that would produce sensible effects on them of a certain kind, and still it could not be supposed that they had true religion, or that they would be saved. Why might not the same thing occur in regard to man?
Thou doest well - So far as this is concerned, or so far as it goes. It is a doctrine which ought to be held, for it is one of the great fundamental truths of religion.
The devils - The "demons," - (τα δαιμόνια ta daimonia). There is, properly, but one being spoken of in the New Testament as "the devil" - ὁ διάβολος ho diabolos, and ὁ Σατᾶν ho Satan - though "demons" are frequently spoken of in the plural number. They are represented as evil spirits, subject to Satan, or under his control, and engaged with him in carrying out his plans of wickedness. These spirits or demons were supposed to wander in desert and desolate places, Matthew 12:43, or to dwell in the atmosphere, (Notes, Ephesians 2:2); they were thought to have the power of working miracles, but not for good, (Revelation 16:14; compare John 10:21); to be hostile to mankind, John 8:44; to utter the pagan oracles, Acts 16:17; to lurk in the idols of the heathen, 1-Corinthians 10:20; and to take up their abodes in the bodies of men, afflicting them with various kinds of diseases, Matthew 7:22; Matthew 9:34; Matthew 10:8; Matthew 17:18; Mark 7:29-30; Luke 4:33; Luke 8:27, Luke 8:30, et soepe. It is of these evil spirits that the apostle speaks when he says that they believe.
Also believe - That is, particularly, they believe in the existence of the one God. How far their knowledge may extend respecting God, we cannot know; but they are never represented in the Scriptures as denying his existence, or as doubting the great truths of religion. They are never described as atheists. That is a sin of this world only. They are not represented as sceptics. That, too, is a peculiar sin of the earth; and probably, in all the universe besides, there are no beings but those who dwell on this globe, who doubt or deny the existence of God, or the other great truths of religion.
And tremble - The word here used (φρίσσουσιν phrissousin) occurs nowhere else in the New Testament. It means, properly, to be rough, uneven, jaggy, sc., with bristling hair; to bristle, to stand on end, as the hair does in a fright; and then to shudder or quake with fear, etc. Here the meaning is, that there was much more in the case referred to than mere speculative faith. There was a faith that produced some effect, and an effect of a very decided character. It did not, indeed, produce good works, or a holy life, but it made it manifest that there was faith; and, consequently, it followed that the existence of mere faith was not all that was necessary to save men, or to make it certain that they would be secure, unless it were held that the devils would be justified and saved by it. If they might hold such faith, and still remain in perdition, men might hold it, and go to perdition. A man should not infer, therefore, because he has faith, even that faith in God which will fill him with alarm, that therefore he is safe. He must have a faith which will produce another effect altogether - that which will lead to a holy life.

Thou believest that there is one God - This is the faith in which these persons put their hope of pleasing God, and of obtaining eternal life. Believing in the being and unity of God distinguished them from all the nations of the world; and having been circumcised, and thus brought into the covenant, they thought themselves secure of salvation. The insufficiency of this St. James immediately shows.
The devils also believe, and tremble - It is well to believe there is one only true God; this truth universal nature proclaims. Even the devils believe it; but far from justifying or saving them, it leaves them in their damned state, and every act of it only increases their torment; φρισσουσι, they shudder with horror, they believe and tremble, are increasingly tormented; but they can neither love nor obey.

(10) Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
(10) Another reason taken from an absurdity: if such a faith were the true faith by means of which we are justified, the demons would be justified, for they have that, but nonetheless they tremble and are not justified, therefore neither is that faith a true faith.

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, &c. 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.
And tremble; at the wrath of God, which they now feel, and at the thought of future torments, which they expect, Mark 5:7 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 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.

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)," Matthew 8:29; Luke 4:34; 2-Peter 2:4; Jde 1:6; Revelation 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 (Hebrews 10:26-27, it is not the faith of love, but of fear, that hath torment, 1-John 4:18).

Thou believest that there is one God. That is very well, but can that alone save you? Even the demons believe that also. See Matthew 8:29. Evil spirits confessed Christ, but this confession of faith did not save them.

Thou believest there is one God - I allow this: but this proves only that thou hast the same faith with the devils. Nay, they not only believe, but tremble - At the dreadful expectation of eternal torments. So far is that faith from either justifying or saving them that have it.