Objection 1. It seems that a created intellect can see the Divine essence by its own natural power. For Dionysius says (Div. Nom. iv): "An angel is a pure mirror, most clear, receiving, if it is right to say so, the whole beauty of God." But if a reflection is seen, the original thing is seen. Therefore since an angel by his natural power understands himself, it seems that by his own natural power he understands the Divine essence.
Objection 2. Further, what is supremely visible, is made less visible to us by reason of our defective corporeal or intellectual sight. But the angelic intellect has no such defect. Therefore, since God is supremely intelligible in Himself, it seems that in like manner He is supremely so to an angel. Therefore, if he can understand other intelligible things by his own natural power, much more can he understand God.
Objection 3. Further, corporeal sense cannot be raised up to understand incorporeal substance, which is above its nature. Therefore if to see the essence of God is above the nature of every created intellect, it follows that no created intellect can reach up to see the essence of God at all. But this is false, as appears from what is said above (Article 1). Therefore it seems that it is natural for a created intellect to see the Divine essence.
On the contrary, It is written: "The grace of God is life everlasting" (Romans 6:23). But life everlasting consists in the vision of the Divine essence, according to the words: "This is eternal life, that they may know Thee the only true God," etc. (John 17:3). Therefore to see the essence of God is possible to the created intellect by grace, and not by nature.
I answer that, It is impossible for any created intellect to see the essence of God by its own natural power. For knowledge is regulated according as the thing known is in the knower. But the thing known is in the knower according to the mode of the knower. Hence the knowledge of every knower is ruled according to its own nature. If therefore the mode of anything's being exceeds the mode of the knower, it must result that the knowledge of the object is above the nature of the knower. Now the mode of being of things is manifold. For some things have being only in this one individual matter; as all bodies. But others are subsisting natures, not residing in matter at all, which, however, are not their own existence, but receive it; and these are the incorporeal beings, called angels. But to God alone does it belong to be His own subsistent being. Therefore what exists only in individual matter we know naturally, forasmuch as our soul, whereby we know, is the form of certain matter. Now our soul possesses two cognitive powers; one is the act of a corporeal organ, which naturally knows things existing in individual matter; hence sense knows only the singular. But there is another kind of cognitive power in the soul, called the intellect; and this is not the act of any corporeal organ. Wherefore the intellect naturally knows natures which exist only in individual matter; not as they are in such individual matter, but according as they are abstracted therefrom by the considering act of the intellect; hence it follows that through the intellect we can understand these objects as universal; and this is beyond the power of the sense. Now the angelic intellect naturally knows natures that are not in matter; but this is beyond the power of the intellect of our soul in the state of its present life, united as it is to the body. It follows therefore that to know self-subsistent being is natural to the divine intellect alone; and this is beyond the natural power of any created intellect; for no creature is its own existence, forasmuch as its existence is participated. Therefore the created intellect cannot see the essence of God, unless God by His grace unites Himself to the created intellect, as an object made intelligible to it.
Reply to Objection 1. This mode of knowing God is natural to an angel--namely, to know Him by His own likeness refulgent in the angel himself. But to know God by any created similitude is not to know the essence of God, as was shown above (Article 2). Hence it does not follow that an angel can know the essence of God by his own power.
Reply to Objection 2. The angelic intellect is not defective, if defect be taken to mean privation, as if it were without anything which it ought to have. But if the defect be taken negatively, in that sense every creature is defective, when compared with God; forasmuch as it does not possess the excellence which is in God.
Reply to Objection 3. The sense of sight, as being altogether material, cannot be raised up to immateriality. But our intellect, or the angelic intellect, inasmuch as it is elevated above matter in its own nature, can be raised up above its own nature to a higher level by grace. The proof is, that sight cannot in any way know abstractedly what it knows concretely; for in no way can it perceive a nature except as this one particular nature; whereas our intellect is able to consider abstractedly what it knows concretely. Now although it knows things which have a form residing in matter, still it resolves the composite into both of these elements; and it considers the form separately by itself. Likewise, also, the intellect of an angel, although it naturally knows the concrete in any nature, still it is able to separate that existence by its intellect; since it knows that the thing itself is one thing, and its existence is another. Since therefore the created intellect is naturally capable of apprehending the concrete form, and the concrete being abstractedly, by way of a kind of resolution of parts; it can by grace be raised up to know separate subsisting substance, and separate subsisting existence.
Source: Summa Theologica (New Advent)