Objection 1. It would seem that the prophets of the demons never foretell the truth. For Ambrose [Hilary the Deacon (Ambrosiaster) on 1 Corinthians 12:3 says that "Every truth, by whomsoever spoken, is from the Holy Ghost." Now the prophets of the demons do not speak from the Holy Ghost, because "there is no concord between Christ and Belial ['What concord hath Christ with Belial?']" (2 Corinthians 6:15). Therefore it would seem that they never foretell the truth.
Objection 2. Further, just as true prophets are inspired by the Spirit of truth, so the prophets of the demons are inspired by the spirit of untruth, according to 1 Kings 22:22, "I will go forth, and be a lying spirit in the mouth of all his prophets." Now the prophets inspired by the Holy Ghost never speak false, as stated above (Question 111, Article 6). Therefore the prophets of the demons never speak truth.
Objection 3. Further, it is said of the devil (John 8:44) that "when he speaketh a lie, he speaketh of his own, for the devil is a liar, and the father thereof," i.e. of lying. Now by inspiring his prophets, the devil speaks only of his own, for he is not appointed God's minister to declare the truth, since "light hath no fellowship with darkness [Vulgate: 'What fellowship hath light with darkness?']" (2 Corinthians 6:14). Therefore the prophets of the demons never foretell the truth.
On the contrary, A gloss on Numbers 22:14, says that "Balaam was a diviner, for he sometimes foreknew the future by help of the demons and the magic art." Now he foretold many true things, for instance that which is to be found in Numbers 24:17: "A star shall rise out of Jacob, and a scepter shall spring up from Israel." Therefore even the prophets of the demons foretell the truth.
I answer that, As the good is in relation to things, so is the true in relation to knowledge. Now in things it is impossible to find one that is wholly devoid of good. Wherefore it is also impossible for any knowledge to be wholly false, without some mixture of truth. Hence Bede says [Comment. in Luc. xvii, 12; Cf. Augustine, QQ. Evang. ii, 40 that "no teaching is so false that it never mingles truth with falsehood." Hence the teaching of the demons, with which they instruct their prophets, contains some truths whereby it is rendered acceptable. For the intellect is led astray to falsehood by the semblance of truth, even as the will is seduced to evil by the semblance of goodness. Wherefore Chrysostom says [Opus Imperf. in Matth., Hom. xix, falsely ascribed to St. John Chrysostom]: "The devil is allowed sometimes to speak true things, in order that his unwonted truthfulness may gain credit for his lie."
Reply to Objection 1. The prophets of the demons do not always speak from the demons' revelation, but sometimes by Divine inspiration. This was evidently the case with Balaam, of whom we read that the Lord spoke to him (Numbers 22:12), though he was a prophet of the demons, because God makes use even of the wicked for the profit of the good. Hence He foretells certain truths even by the demons' prophets, both that the truth may be rendered more credible, since even its foes bear witness to it, and also in order that men, by believing such men, may be more easily led on to truth. Wherefore also the Sibyls foretold many true things about Christ.
Yet even when the demons' prophets are instructed by the demons, they foretell the truth, sometimes by virtue of their own nature, the author of which is the Holy Ghost, and sometimes by revelation of the good spirits, as Augustine declares (Gen. ad lit. xii, 19): so that even then this truth which the demons proclaim is from the Holy Ghost.
Reply to Objection 2. A true prophet is always inspired by the Spirit of truth, in Whom there is no falsehood, wherefore He never says what is not true; whereas a false prophet is not always instructed by the spirit of untruth, but sometimes even by the Spirit of truth. Even the very spirit of untruth sometimes declares true things, sometimes false, as stated above.
Reply to Objection 3. Those things are called the demons' own, which they have of themselves, namely lies and sins; while they have, not of themselves but of God, those things which belong to them by nature: and it is by virtue of their own nature that they sometimes foretell the truth, as stated above (ad 1). Moreover God makes use of them to make known the truth which is to be accomplished through them, by revealing Divine mysteries to them through the angels, as already stated (Gen. ad lit. xii, 19; I, 109, 4, ad 1).
Source: Summa Theologica (New Advent)