Objection 1. It would seem that there can be falsehood in the angel's intellect. For perversity appertains to falsehood. But, as Dionysius says (Div. Nom. iv), there is "a perverted fancy" in the demons. Therefore it seems that there can be falsehood in the intellect of the angels.
Objection 2. Further, nescience is the cause of estimating falsely. But, as Dionysius says (Eccl. Hier. vi), there can be nescience in the angels. Therefore it seems there can be falsehood in them.
Objection 3. Further, everything which falls short of the truth of wisdom, and which has a depraved reason, has falsehood or error in its intellect. But Dionysius (Div. Nom. vii) affirms this of the demons. Therefore it seems that there can be error in the minds of the angels.
On the contrary, The Philosopher says (De Anima iii, text. 41) that "the intelligence is always true." Augustine likewise says (QQ. 83, qu. 32) that "nothing but what is true can be the object of intelligence" Therefore there can be neither deception nor falsehood in the angel's knowledge.
I answer that, The truth of this question depends partly upon what has gone before. For it has been said (4) that an angel understands not by composing and dividing, but by understanding what a thing is. Now the intellect is always true as regards what a thing is, just as the sense regarding its proper object, as is said in De Anima iii, text. 26. But by accident, deception and falsehood creep in, when we understand the essence of a thing by some kind of composition, and this happens either when we take the definition of one thing for another, or when the parts of a definition do not hang together, as if we were to accept as the definition of some creature, "a four-footed flying beast," for there is no such animal. And this comes about in things composite, the definition of which is drawn from diverse elements, one of which is as matter to the other. But there is no room for error in understanding simple quiddities, as is stated in Metaph. ix, text. 22; for either they are not grasped at all, and so we know nothing respecting them; or else they are known precisely as they exist.
So therefore, no falsehood, error, or deception can exist of itself in the mind of any angel; yet it does so happen accidentally; but very differently from the way it befalls us. For we sometimes get at the quiddity of a thing by a composing and dividing process, as when, by division and demonstration, we seek out the truth of a definition. Such is not the method of the angels; but through the (knowledge of the) essence of a thing they know everything that can be said regarding it. Now it is quite evident that the quiddity of a thing can be a source of knowledge with regard to everything belonging to such thing, or excluded from it; but not of what may be dependent on God's supernatural ordinance. Consequently, owing to their upright will, from their knowing the nature of every creature, the good angels form no judgments as to the nature of the qualities therein, save under the Divine ordinance; hence there can be no error or falsehood in them. But since the minds of demons are utterly perverted from the Divine wisdom, they at times form their opinions of things simply according to the natural conditions of the same. Nor are they ever deceived as to the natural properties of anything; but they can be misled with regard to supernatural matters; for example, on seeing a dead man, they may suppose that he will not rise again, or, on beholding Christ, they may judge Him not to be God.
From all this the answers to the objections of both sides of the question are evident. For the perversity of the demons comes of their not being subject to the Divine wisdom; while nescience is in the angels as regards things knowable, not naturally but supernaturally. It is, furthermore, evident that their understanding of what a thing is, is always true, save accidentally, according as it is, in an undue manner, referred to some composition or division.
Source: Summa Theologica (New Advent)