Objection 1. It seems that the created intellect does not need any created light in order to see the essence of God. For what is of itself lucid in sensible things does not require any other light in order to be seen. Therefore the same applies to intelligible things. Now God is intelligible light. Therefore He is not seen by means of any created light.
Objection 2. Further, if God is seen through a medium, He is not seen in His essence. But if seen by any created light, He is seen through a medium. Therefore He is not seen in His essence.
Objection 3. Further, what is created can be natural to some creature. Therefore if the essence of God is seen through any created light, such a light can be made natural to some other creature; and thus, that creature would not need any other light to see God; which is impossible. Therefore it is not necessary that every creature should require a superadded light in order to see the essence of God.
On the contrary, It is written: "In Thy light we shall see light" (Psalm 35:10).
I answer that, Everything which is raised up to what exceeds its nature, must be prepared by some disposition above its nature; as, for example, if air is to receive the form of fire, it must be prepared by some disposition for such a form. But when any created intellect sees the essence of God, the essence of God itself becomes the intelligible form of the intellect. Hence it is necessary that some supernatural disposition should be added to the intellect in order that it may be raised up to such a great and sublime height. Now since the natural power of the created intellect does not avail to enable it to see the essence of God, as was shown in the preceding article, it is necessary that the power of understanding should be added by divine grace. Now this increase of the intellectual powers is called the illumination of the intellect, as we also call the intelligible object itself by the name of light of illumination. And this is the light spoken of in the Apocalypse (Apocalypse 21:23): "The glory of God hath enlightened it"--viz. the society of the blessed who see God. By this light the blessed are made "deiform"--i.e. like to God, according to the saying: "When He shall appear we shall be like to Him, and [Vulgate: 'because'] we shall see Him as He is" (1 John 2:2).
Reply to Objection 1. The created light is necessary to see the essence of God, not in order to make the essence of God intelligible, which is of itself intelligible, but in order to enable the intellect to understand in the same way as a habit makes a power abler to act. Even so corporeal light is necessary as regards external sight, inasmuch as it makes the medium actually transparent, and susceptible of color.
Reply to Objection 2. This light is required to see the divine essence, not as a similitude in which God is seen, but as a perfection of the intellect, strengthening it to see God. Therefore it may be said that this light is to be described not as a medium in which God is seen, but as one by which He is seen; and such a medium does not take away the immediate vision of God.
Reply to Objection 3. The disposition to the form of fire can be natural only to the subject of that form. Hence the light of glory cannot be natural to a creature unless the creature has a divine nature; which is impossible. But by this light the rational creature is made deiform, as is said in this article.
Source: Summa Theologica (New Advent)