Objection 1. It seems that no created intellect can see the essence of God. For Chrysostom (Hom. xiv. in Joan.) commenting on John 1:18, "No man hath seen God at any time," says: "Not prophets only, but neither angels nor archangels have seen God. For how can a creature see what is increatable?" Dionysius also says (Div. Nom. i), speaking of God: "Neither is there sense, nor image, nor opinion, nor reason, nor knowledge of Him."
Objection 2. Further, everything infinite, as such, is unknown. But God is infinite, as was shown above (Question 7, Article 1). Therefore in Himself He is unknown.
Objection 3. Further, the created intellect knows only existing things. For what falls first under the apprehension of the intellect is being. Now God is not something existing; but He is rather super-existence, as Dionysius says (Div. Nom. iv). Therefore God is not intelligible; but above all intellect.
Objection 4. Further, there must be some proportion between the knower and the known, since the known is the perfection of the knower. But no proportion exists between the created intellect and God; for there is an infinite distance between them. Therefore the created intellect cannot see the essence of God.
On the contrary, It is written: "We shall see Him as He is" (1 John 2:2).
I answer that, Since everything is knowable according as it is actual, God, Who is pure act without any admixture of potentiality, is in Himself supremely knowable. But what is supremely knowable in itself, may not be knowable to a particular intellect, on account of the excess of the intelligible object above the intellect; as, for example, the sun, which is supremely visible, cannot be seen by the bat by reason of its excess of light.
Therefore some who considered this, held that no created intellect can see the essence of God. This opinion, however, is not tenable. For as the ultimate beatitude of man consists in the use of his highest function, which is the operation of his intellect; if we suppose that the created intellect could never see God, it would either never attain to beatitude, or its beatitude would consist in something else beside God; which is opposed to faith. For the ultimate perfection of the rational creature is to be found in that which is the principle of its being; since a thing is perfect so far as it attains to its principle. Further the same opinion is also against reason. For there resides in every man a natural desire to know the cause of any effect which he sees; and thence arises wonder in men. But if the intellect of the rational creature could not reach so far as to the first cause of things, the natural desire would remain void.
Hence it must be absolutely granted that the blessed see the essence of God.
Reply to Objection 1. Both of these authorities speak of the vision of comprehension. Hence Dionysius premises immediately before the words cited, "He is universally to all incomprehensible," etc. Chrysostom likewise after the words quoted says: "He says this of the most certain vision ofthe Father, which is such a perfect consideration and comprehension as the Father has of the Son."
Reply to Objection 2. The infinity of matter not made perfect by form, is unknown in itself, because all knowledge comes by the form; whereas the infinity of the form not limited by matter, is in itself supremely known. God is Infinite in this way, and not in the first way: as appears from what was said above (Question 7, Article 1).
Reply to Objection 3. God is not said to be not existing as if He did not exist at all, but because He exists above all that exists; inasmuch as He is His own existence. Hence it does not follow that He cannot be known at all, but that He exceeds every kind of knowledge; which means that He is not comprehended.
Reply to Objection 4. Proportion is twofold. In one sense it means a certain relation of one quantity to another, according as double, treble and equal are species of proportion. In another sense every relation of one thing to another is called proportion. And in this sense there can be a proportion of the creature to God, inasmuch as it is related to Him as the effect of its cause, and as potentiality to its act; and in this way the created intellect can be proportioned to know God.
Source: Summa Theologica (New Advent)