Objection 1. It seems that the essence of God is seen through an image by the created intellect. For it is written: "We know that when He shall appear, we shall be like to Him, and [Vulgate: 'because'] we shall see Him as He is" (1 John 3:2).
Objection 2. Further, Augustine says (De Trin. v): "When we know God, some likeness of God is made in us."
Objection 3. Further, the intellect in act is the actual intelligible; as sense in act is the actual sensible. But this comes about inasmuch as sense is informed with the likeness of the sensible object, and the intellect with the likeness of the thing understood. Therefore, if God is seen by the created intellect in act, it must be that He is seen by some similitude.
On the contrary, Augustine says (De Trin. xv) that when the Apostle says, "We see through a glass and in an enigma [Douay: 'in a dark manner']," "by the terms 'glass' and 'enigma' certain similitudes are signified by him, which are accommodated to the vision of God." But to see the essence of God is not an enigmatic nor a speculative vision, but is, on the contrary, of an opposite kind. Therefore the divine essence is not seen through a similitude.
I answer that, Two things are required both for sensible and for intellectual vision--viz. power of sight, and union of the thing seen with the sight. For vision is made actual only when the thing seen is in a certain way in the seer. Now in corporeal things it is clear that the thing seen cannot be by its essence in the seer, but only by its likeness; as the similitude of a stone is in the eye, whereby the vision is made actual; whereas the substance of the stone is not there. But if the principle of the visual power and the thing seen were one and the same thing, it would necessarily follow that the seer would receive both the visual power and the form whereby it sees, from that one same thing.
Now it is manifest both that God is the author of the intellect power, and that He can be seen by the intellect. And since the intellective power of the creature is not the essence of God, it follows that it is some kind of participated likeness of Him who is the first intellect. Hence also the intellectual power of the creature is called an intelligible light, as it were, derived from the first light, whether this be understood of the natural power, or of some perfection superadded of grace or of glory. Therefore, in order to see God, there must be some similitude of God on the part of the visual faculty, whereby the intellect is made capable of seeing God. But on the part of the object seen, which must necessarily be united to the seer, the essence of God cannot be seen by any created similitude.
First, because as Dionysius says (Div. Nom. i), "by the similitudes of the inferior order of things, the superior can in no way be known;" as by the likeness of a body the essence of an incorporeal thing cannot be known. Much less therefore can the essence of God be seen by any created likeness whatever.
Secondly, because the essence of God is His own very existence, as was shown above (Question 3, Article 4), which cannot be said of any created form; and so no created form can be the similitude representing the essence of God to the seer.
Thirdly, because the divine essence is uncircumscribed, and contains in itself super-eminently whatever can be signified or understood by the created intellect. Now this cannot in any way be represented by any created likeness; for every created form is determined according to some aspect of wisdom, or of power, or of being itself, or of some like thing. Hence to say that God is seen by some similitude, is to say that the divine essence is not seen at all; which is false.
Therefore it must be said that to see the essence of God, there is required some similitude in the visual faculty, namely, the light of glory strengthening the intellect to see God, which is spoken of in the Psalm 35:10, "In Thy light we shall see light." The essence of God, however, cannot be seen by any created similitude representing the divine essence itself as it really is.
Reply to Objection 1. That authority speaks of the similitude which is caused by participation of the light of glory.
Reply to Objection 2. Augustine speaks of the knowledge of God here on earth.
Reply to Objection 3. The divine essence is existence itself. Hence as other intelligible forms which are not their own existence are united to the intellect by means of some entity, whereby the intellect itself is informed, and made in act; so the divine essence is united to the created intellect, as the object actually understood, making the intellect in act by and of itself.
Source: Summa Theologica (New Advent)