Objection 1. It would seem that the demons' intellect is darkened by being deprived of the knowledge of all truth. For it they knew any truth at all, they would most of all know themselves; which is to know separated substances. But this is not in keeping with their unhappiness: for this seems to belong to great happiness, insomuch as that some writers have assigned as man's last happiness the knowledge of the separated substances. Therefore the demons are deprived of all knowledge of truth.
Objection 2. Further, what is most manifest in its nature, seems to be specially manifest to the angels, whether good or bad. That the same is not manifest with regard to ourselves, comes from the weakness of our intellect which draws its knowledge from phantasms; as it comes from the weakness of its eye that the owl cannot behold the light of the sun. But the demons cannot know God, Who is most manifest of Himself, because He is the sovereign truth; and this is because they are not clean of heart, whereby alone can God be seen. Therefore neither can they know other things.
Objection 3. Further, according to Augustine (Gen. ad lit. iv, 22), the proper knowledge of the angels is twofold; namely, morning and evening. But the demons have no morning knowledge, because they do not see things in the Word; nor have they the evening knowledge, because this evening knowledge refers the things known to the Creator's praise (hence, after "evening" comes "morning" [Genesis 1]). Therefore the demons can have no knowledge of things.
Objection 4. Further, the angels at their creation knew the mystery of the kingdom of God, as Augustine says (Gen. ad lit. v, 19; De Civ. Dei xi). But the demons are deprived of such knowledge: "for if they had known it, they would never have crucified the Lord of glory," as is said 1 Corinthians 2:8. Therefore, for the same reason, they are deprived of all other knowledge of truth.
Objection 5. Further, whatever truth anyone knows is known either naturally, as we know first principles; or by deriving it from someone else, as we know by learning; or by long experience, as the things we learn by discovery. Now, the demons cannot know the truth by their own nature, because, as Augustine says (De Civ. Dei xi, 33), the good angels are separated from them as light is from darkness; and every manifestation is made through light, as is said Ephesians 5:13. In like manner they cannot learn by revelation, nor by learning from the good angels: because "there is no fellowship of light with darkness [Vulgate: 'What fellowship hath...?']" (2 Corinthians 6:14). Nor can they learn by long experience: because experience comes of the senses. Consequently there is no knowledge of truth in them.
On the contrary, Dionysius says (Div. Nom. iv) that, "certain gifts were bestowed upon the demons which, we say, have not been changed at all, but remain entire and most brilliant." Now, the knowledge of truth stands among those natural gifts. Consequently there is some knowledge of truth in them.
I answer that, The knowledge of truth is twofold: one which comes of nature, and one which comes of grace. The knowledge which comes of grace is likewise twofold: the first is purely speculative, as when Divine secrets are imparted to an individual; the other is effective, and produces love for God; which knowledge properly belongs to the gift of wisdom.
Of these three kinds of knowledge the first was neither taken away nor lessened in the demons. For it follows from the very nature of the angel, who, according to his nature, is an intellect or mind: since on account of the simplicity of his substance, nothing can be withdrawn from his nature, so as to punish him by subtracting from his natural powers, as a man is punished by being deprived of a hand or a foot or of something else. Therefore Dionysius says (Div. Nom. iv) that the natural gifts remain entire in them. Consequently their natural knowledge was not diminished. The second kind of knowledge, however, which comes of grace, and consists in speculation, has not been utterly taken away from them, but lessened; because, of these Divine secrets only so much is revealed to them as is necessary; and that is done either by means of the angels, or "through some temporal workings of Divine power," as Augustine says (De Civ. Dei ix, 21); but not in the same degree as to the holy angels, to whom many more things are revealed, and more fully, in the Word Himself. But of the third knowledge, as likewise of charity, they are utterly deprived.
Reply to Objection 1. Happiness consists in self-application to something higher. The separated substances are above us in the order of nature; hence man can have happiness of a kind by knowing the separated substances, although his perfect happiness consists in knowing the first substance, namely, God. But it is quite natural for one separate substance to know another; as it is natural for us to know sensible natures. Hence, as man's happiness does not consist in knowing sensible natures; so neither does the angel's happiness consist in knowing separated substances.
Reply to Objection 2. What is most manifest in its nature is hidden from us by its surpassing the bounds of our intellect; and not merely because our intellect draws knowledge from phantasms. Now the Divine substance surpasses the proportion not only of the human intellect, but even of the angelic. Consequently, not even an angel can of his own nature know God's substance. Yet on account of the perfection of his intellect he can of his nature have a higher knowledge of God than man can have. Such knowledge of God remains also in the demons. Although they do not possess the purity which comes with grace, nevertheless they have purity of nature; and this suffices for the knowledge of God which belongs to them from their nature.
Reply to Objection 3. The creature is darkness in comparison with the excellence of the Divine light; and therefore the creature's knowledge in its own nature is called "evening" knowledge. For the evening is akin to darkness, yet it possesses some light: but when the light fails utterly, then it is night. So then the knowledge of things in their own nature, when referred to the praise of the Creator, as it is in the good angels, has something of the Divine light, and can be called evening knowledge; but if it be not referred to God, as is the case with the demons, it is not called evening, but "nocturnal" knowledge. Accordingly we read in Genesis 1:5 that the darkness, which God separated from the light, "He called night."
Reply to Objection 4. All the angels had some knowledge from the very beginning respecting the mystery of God's kingdom, which found its completion in Christ; and most of all from the moment when they were beatified by the vision of the Word, which vision the demons never had. Yet all the angels did not fully and equally apprehend it; hence the demons much less fully understood the mystery of the Incarnation, when Christ was in the world. For, as Augustine observes (De Civ. Dei ix, 21), "It was not manifested to them as it was to the holy angels, who enjoy a participated eternity of the Word; but it was made known by some temporal effects, so as to strike terror into them." For had they fully and certainly known that He was the Son of God and the effect of His passion, they would never have procured the crucifixion of the Lord of glory.
Reply to Objection 5. The demons know a truth in three ways: first of all by the subtlety of their nature; for although they are darkened by privation of the light of grace, yet they are enlightened by the light of their intellectual nature: secondly, by revelation from the holy angels; for while not agreeing with them in conformity of will, they do agree, nevertheless, by their likeness of intellectual nature, according to which they can accept what is manifested by others: thirdly, they know by long experience; not as deriving it from the senses; but when the similitude of their innate intelligible species is completed in individual things, they know some things as present, which they previously did not know would come to pass, as we said when dealing with the knowledge of the angels (57, 3, ad 3).
Source: Summa Theologica (New Advent)