Objection 1. It would seem that the angels know all things by their substance. For Dionysius says (Div. Nom. vii) that "the angels, according to the proper nature of a mind, know the things which are happening upon earth." But the angel's nature is his essence. Therefore the angel knows things by his essence.
Objection 2. Further, according to the Philosopher (Metaph. xii, text. 51; De Anima iii, text. 15), "in things which are without matter, the intellect is the same as the object understood." But the object understood is the same as the one who understands it, as regards that whereby it is understood. Therefore in things without matter, such as the angels, the medium whereby the object is understood is the very substance of the one understanding it.
Objection 3. Further, everything which is contained in another is there according to the mode of the container. But an angel has an intellectual nature. Therefore whatever is in him is there in an intelligible mode. But all things are in him: because the lower orders of beings are essentially in the higher, while the higher are in the lower participatively: and therefore Dionysius says (Div. Nom. iv) that God "enfolds the whole in the whole," i.e. all in all. Therefore the angel knows all things in his substance.
On the contrary, Dionysius says (Div. Nom. iv) that "the angels are enlightened by the forms of things." Therefore they know by the forms of things, and not by their own substance.
I answer that, The medium through which the intellect understands, is compared to the intellect understanding it as its form, because it is by the form that the agent acts. Now in order that the faculty may be perfectly completed by the form, it is necessary for all things to which the faculty extends to be contained under the form. Hence it is that in things which are corruptible, the form does not perfectly complete the potentiality of the matter: because the potentiality of the matter extends to more things than are contained under this or that form. But the intellective power of the angel extends to understanding all things: because the object of the intellect is universal being or universal truth. The angel's essence, however, does not comprise all things in itself, since it is an essence restricted to a genus and species. This is proper to the Divine essence, which is infinite, simply and perfectly to comprise all things in Itself. Therefore God alone knows all things by His essence. But an angel cannot know all things by his essence; and his intellect must be perfected by some species in order to know things.
Reply to Objection 1. When it is said that the angel knows things according to his own nature, the words "according to" do not determine the medium of such knowledge, since the medium is the similitude of the thing known; but they denote the knowing power, which belongs to the angel of his own nature.
Reply to Objection 2. As the sense in act is the sensible in act, as stated in De Anima ii, text. 53, not so that the sensitive power is the sensible object's likeness contained in the sense, but because one thing is made from both as from act and potentiality: so likewise the intellect in act is said to be the thing understood in act, not that the substance of the intellect is itself the similitude by which it understands, but because that similitude is its form. Now, it is precisely the same thing to say "in things which are without matter, the intellect is the same thing as the object understood," as to say that "the intellect in act is the thing understood in act"; for a thing is actually understood, precisely because it is immaterial.
Reply to Objection 3. The things which are beneath the angel, and those which are above him, are in a measure in his substance, not indeed perfectly, nor according to their own proper formality--because the angel's essence, as being finite, is distinguished by its own formality from other things--but according to some common formality. Yet all things are perfectly and according to their own formality in God's essence, as in the first and universal operative power, from which proceeds whatever is proper or common to anything. Therefore God has a proper knowledge of all things by His own essence: and this the angel has not, but only a common knowledge.
Source: Summa Theologica (New Advent)