Objection 1. It seems that of those who see the essence of God, one does not see more perfectly than another. For it is written (1 John 3:2): "We shall see Him as He is." But He is only in one way. Therefore He will be seen by all in one way only; and therefore He will not be seen more perfectly by one and less perfectly by another.
Objection 2. Further, Augustine says (Octog. Tri. Quaest. qu. xxxii): "One person cannot see one and the same thing more perfectly than another." But all who see the essence of God, understand the Divine essence, for God is seen by the intellect and not by sense, as was shown above (Article 3). Therefore of those who see the divine essence, one does not see more clearly than another.
Objection 3. Further, That anything be seen more perfectly than another can happen in two ways: either on the part of the visible object, or on the part of the visual power of the seer. On the part of the object, it may so happen because the object is received more perfectly in the seer, that is, according to the greater perfection of the similitude; but this does not apply to the present question, for God is present to the intellect seeing Him not by way of similitude, but by His essence. It follows then that if one sees Him more perfectly than another, this happens according to the difference of the intellectual power; thus it follows too that the one whose intellectual power is higher, will see Him the more clearly; and this is incongruous; since equality with angels is promised to men as their beatitude.
On the contrary, Eternal life consists in the vision of God, according to John 17:3: "This is eternal life, that they may know Thee the only true God," etc. Therefore if all saw the essence of God equally in eternal life, all would be equal; the contrary to which is declared by the Apostle: "Star differs from star in glory" (1 Corinthians 15:41).
I answer that, Of those who see the essence of God, one sees Him more perfectly than another. This, indeed, does not take place as if one had a more perfect similitude of God than another, since that vision will not spring from any similitude; but it will take place because one intellect will have a greater power or faculty to see God than another. The faculty of seeing God, however, does not belong to the created intellect naturally, but is given to it by the light of glory, which establishes the intellect in a kind of "deiformity," as appears from what is said above, in the preceding article.
Hence the intellect which has more of the light of glory will see God the more perfectly; and he will have a fuller participation of the light of glory who has more charity; because where there is the greater charity, there is the more desire; and desire in a certain degree makes the one desiring apt and prepared to receive the object desired. Hence he who possesses the more charity, will see God the more perfectly, and will be the more beatified.
Reply to Objection 1. In the words,"We shall see Him as He is," the conjunction "as" determines the mode of vision on the part of the object seen, so that the meaning is, we shall see Him to be as He is, because we shall see His existence, which is His essence. But it does not determine the mode of vision on the part of the one seeing; as if the meaning was that the mode of seeing God will be as perfect as is the perfect mode of God's existence.
Thus appears the answer to the Second Objection. For when it is said that one intellect does not understand one and the same thing better than another, this would be true if referred to the mode of the thing understood, for whoever understands it otherwise than it really is, does not truly understand it, but not if referred to the mode of understanding, for the understanding of one is more perfect than the understanding of another.
Reply to Objection 3. The diversity of seeing will not arise on the part of the object seen, for the same object will be presented to all--viz. the essence of God; nor will it arise from the diverse participation of the object seen by different similitudes; but it will arise on the part of the diverse faculty of the intellect, not, indeed, the natural faculty, but the glorified faculty.
Source: Summa Theologica (New Advent)