You believe that there is one God; you do well: the demons also believe, and tremble.
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Andreas of Caesarea
AD 637
James gives us the example of the devils, saying that those who profess faith with their lips only are really no better than they are. For even they believe that Christ is the Son of God, that he is the Holy One of God and that he has authority over them.
Will the devils see God? Those who are pure of heart will see him, and who would say that the devils are pure of heart? Nevertheless, they believe and tremble. (Commentary on the Sermon on the Mount 53.10)
Those who believe and act according to true faith do live and are not dead, but those who do not believe, or else who believe like the demons, trembling but living evilly, proclaiming the Son of God but not having love, must rather be accounted dead. (Tractates 22.7.2)
You can believe what God says, you can believe that God exists, and you can believe in him, which means that you love him so much that you want to do what he tells you. There are many evil people around who can manage the first two of these. They believe that God means what he says, and they are quite prepared to accept that he exists. But it takes someone who is not just a nominal Christian but who is one in deed and in living to love God and to do what he commands. Faith with love is Christian, but faith without love is demonic.
The apostle says that a man who believes and does not act has the faith of demons. If that is true, imagine the fate of a man who does not believe at all.
The devils also believe, and tremble. St. James compares indeed faith without other virtues and good works, to the faith of devils: but comparisons must never be stretched farther than they are intended. The meaning is, that such a faith in sinners is unprofitable to salvation, like that of devils, which is no more than a conviction from their knowledge of God; but faith which remains in sinners, is from a supernatural knowledge, together with a pious motion in their free will. (Witham)
Those who believe but who do not fear God are even worse than the devils. And those who believe and tremble but who do not practice what they preach are just like the devils.
Good works are witnesses to the Christian faith, because otherwise a Christian cannot prove that he has that faith. If he cannot prove it, it must be considered completely nonexistent.