What do demons believe anyway?

"The Puritan writer Thomas Manton in his excellent commentary on the Epistle of James gives a characteristic and significant answer. The paragraph in question discusses James 2:19, 'The devils also believe, and tremble.' 'This instance showeth,' says Manton, 'what faith he disputeth against, namely, such as consisteth in bare speculation and knowledge. . . .Thou believest; that is assentest to this truth; the lowest act of faith is invested with the name of believing.'
Manton's argument here is that since the devils assent and true believers also assent, something other than assent is needed for saving faith. This is a logical blunder. The text says the devils believe in monotheism. Why cannot the difference between the devils and Christians be the different propositions believed, rather than a psychological element in belief? Manton assumes a different psychology is needed. It is better to say a different object of belief is needed.
According to Manton, the devils' psychology is one of bare speculation. However, he does not explain what this is. If it is the so-called faith, discussed on his preceding pages, that produces no works, one cannot object. This so-called faith Manton calls a 'dead faith,' or better a 'false faith,' and therefore not a saving faith at all. Faith without works is dead. Agreed. But if this is not saving faith at all, and is yet called faith and belief, the difference will be found in the object, not in the psychological analysis. The analysis is the same whether a person believes a saving truth, a non-saving truth, or even a falsehood.
Manton makes an attempt to avoid the force of this consideration. 'There is one God,' he continues. 'He instanceth in this proposition, though he doth limit the matter only to this: partly because this was the first article of the creed . . . by it intending also assent to other articles of religion . . . .'
Now, just what devils believe and do not believe, the Bible does not fully explain. The psychology of Satan is something of a puzzle. Apparently Satan really believed that Job would curse God. Like the Arminians he did not believe in the persevereance of the saints. One cannot be certain, but possibly Satan believed the promise he himself made to Eve. Did he not also believe that he might possibly tempt Christ to sin? If he had believed it impossible, why should he have tried three times? There must therefore be a good bit of the Bible that the devils do not believe.
In this difficulty it is best to stay close to the text, and James says only that the devils believe there is one God. The text nowhere says that this proposition stands for all the articles of the creed. It has just now been proved that it does not. If human beings can be monotheists without believing in the atonement, or even in Christ, one might suppose the devils could too. Because Manton adds to the creed of the devils propositions James does not specify, his argument becomes confused. Depending on an hypothesis that has no textual foundation, he fails to escape the objection above: It is illogical to conclude that belief is not assent just because belief in monotheism does not save. The clearer inference is that if belief in monotheism does not save, then one ought to believe something else in addition. Not assent, but monotheism is inadequate." Gordon H. Clark, What is Saving Faith?


John Calvin also says, "When the apostle gives the appellation of faith to a vain notion, widely different from true faith, it is a concession which derogates nothing from the argument; this he shows from the beginning in these words: 'What doth it profit, my brethren, though a man say he hath faith, and have not works?' He does not say, If any one have faith without works; but If any one boast of having it. He speaks speaks more plainly just after, where he ridicules it by representing it as worse than the knowledge of devils; and lastly, when he calls it dead. But his meaning may be sufficiently understood from the definition he gives: 'Thou believest,' says he, 'that there is one God.' Indeed, if nothing be contained in this creed but a belief of the Divine existence, it is not at all surprising that it is inadequate to justification. And we must not suppose this denial to be derogatory to Christian faith, the nature of which is widely different. For how does true faith justify, but by uniting us to Christ, that, being made one with him, we may participate his righteousness? It does not, therefore, justify us, by attaining a knowledge of God's existence, but by a reliance on the certainty of his mercy." - The Institutes, 3. 17. 11. Pg. 45-46