Objection 1. It would seem that no prophecy comes from the demons. For prophecy is "a Divine revelation," according to Cassiodorus [Prol. in Psalt. i]. But that which is done by a demon is not Divine. Therefore no prophecy can be from a demon.
Objection 2. Further, some kind of enlightenment is requisite for prophetic knowledge, as stated above (171, 2,3). Now the demons do not enlighten the human intellect, as stated above in I, 119, 3. Therefore no prophecy can come from the demons.
Objection 3. Further, a sign is worthless if it betokens contraries. Now prophecy is a sign in confirmation of faith; wherefore a gloss on Romans 12:6, "Either prophecy to be used according to the rule of faith," says: "Observe that in reckoning the graces, he begins with prophecy, which is the first proof of the reasonableness of our faith; since believers, after receiving the Spirit, prophesied." Therefore prophecy cannot be bestowed by the demons.
On the contrary, It is written (1 Kings 18:19): "Gather unto me all Israel unto mount Carmel, and the prophets of Baal four hundred and fifty, and the prophets of the grove four hundred, who eat at Jezebel's table." Now these were worshippers of demons. Therefore it would seem that there is also a prophecy from the demons.
I answer that, As stated above (Question 171, Article 1), prophecy denotes knowledge far removed from human knowledge. Now it is evident that an intellect of a higher order can know some things that are far removed from the knowledge of an inferior intellect. Again, above the human intellect there is not only the Divine intellect, but also the intellects of good and bad angels according to the order of nature. Hence the demons, even by their natural knowledge, know certain things remote from men's knowledge, which they can reveal to men: although those things which God alone knows are remote simply and most of all.
Accordingly prophecy, properly and simply, is conveyed by Divine revelations alone; yet the revelation which is made by the demons may be called prophecy in a restricted sense. Wherefore those men to whom something is revealed by the demons are styled in the Scriptures as prophets, not simply, but with an addition, for instance as "false prophets," or "prophets of idols." Hence Augustine says (Gen. ad lit. xii, 19): "When the evil spirit lays hold of a man for such purposes as these," namely visions, "he makes him either devilish, or possessed, or a false prophet."
Reply to Objection 1. Cassiodorus is here defining prophecy in its proper and simple acceptation.
Reply to Objection 2. The demons reveal what they know to men, not by enlightening the intellect, but by an imaginary vision, or even by audible speech; and in this way this prophecy differs from true prophecy.
Reply to Objection 3. The prophecy of the demons can be distinguished from Divine prophecy by certain, and even outward, signs. Hence Chrysostom says [Opus Imperf. in Matth., Hom. xix, falsely ascribed to St. John Chrysostom] that "some prophesy by the spirit of the devil, such as diviners, but they may be discerned by the fact that the devil sometimes utters what is false, the Holy Ghost never." Wherefore it is written (Deuteronomy 18:21-22): "If in silent thought thou answer: How shall I know the word that the Lord hath spoken? Thou shalt have this sign: Whatsoever that same prophet foretelleth in the name of the Lord, and it come not to pass, that thing the Lord hath not spoken."
Source: Summa Theologica (New Advent)