Objection 1. It would seem that the prophets see the very essence of God, for a gloss on Isaiah 38:1, "Take order with thy house, for thou shalt die and not live," says: "Prophets can read in the book of God's foreknowledge in which all things are written." Now God's foreknowledge is His very essence. Therefore prophets see God's very essence.
Objection 2. Further, Augustine says (De Trin. ix, 7) that "in that eternal truth from which all temporal things are made, we see with the mind's eye the type both of our being and of our actions." Now, of all men, prophets have the highest knowledge of Divine things. Therefore they, especially, see the Divine essence.
Objection 3. Further, future contingencies are foreknown by the prophets "with unchangeable truth." Now future contingencies exist thus in God alone. Therefore the prophets see God Himself.
On the contrary, The vision of the Divine essence is not made void in heaven; whereas "prophecy is made void" (1 Corinthians 13:8). Therefore prophecy is not conveyed by a vision of the Divine essence.
I answer that, Prophecy denotes Divine knowledge as existing afar off. Wherefore it is said of the prophets (Hebrews 11:13) that "they were beholding... afar off." But those who are in heaven and in the state of bliss see, not as from afar off, but rather, as it were, from near at hand, according to Psalm 139:14, "The upright shall dwell with Thy countenance." Hence it is evident that prophetic knowledge differs from the perfect knowledge, which we shall have in heaven, so that it is distinguished therefrom as the imperfect from the perfect, and when the latter comes the former is made void, as appears from the words of the Apostle (1 Corinthians 13:10).
Some, however, wishing to discriminate between prophetic knowledge and the knowledge of the blessed, have maintained that the prophets see the very essence of God (which they call the "mirror of eternity") [Cf. De Veritate, xii, 6; Sent. II, D, XI, part 2, art. 2, ad 4, not, however, in the way in which it is the object of the blessed, but as containing the types [Cf. I, 15] of future events. But this is altogether impossible. For God is the object of bliss in His very essence, according to the saying of Augustine (Confess. v, 4): "Happy whoso knoweth Thee, though he know not these," i.e. creatures. Now it is not possible to see the types of creatures in the very essence of God without seeing It, both because the Divine essence is Itself the type of all things that are made--the ideal type adding nothing to the Divine essence save only a relationship to the creature--and because knowledge of a thing in itself--and such is the knowledge of God as the object of heavenly bliss--precedes knowledge of that thing in its relation to something else--and such is the knowledge of God as containing the types of things. Consequently it is impossible for prophets to see God as containing the types of creatures, and yet not as the object of bliss. Therefore we must conclude that the prophetic vision is not the vision of the very essence of God, and that the prophets do not see in the Divine essence Itself the things they do see, but that they see them in certain images, according as they are enlightened by the Divine light.
Wherefore Dionysius (Coel. Hier. iv), in speaking of prophetic visions, says that "the wise theologian calls that vision divine which is effected by images of things lacking a bodily form through the seer being rapt in divine things." And these images illumined by the Divine light have more of the nature of a mirror than the Divine essence: since in a mirror images are formed from other things, and this cannot be said of God. Yet the prophet's mind thus enlightened may be called a mirror, in so far as a likeness of the truth of the Divine foreknowledge is formed therein, for which reason it is called the "mirror of eternity," as representing God's foreknowledge, for God in His eternity sees all things as present before Him, as stated above (Question 172, Article 1).
Reply to Objection 1. The prophets are said to read the book of God's foreknowledge, inasmuch as the truth is reflected from God's foreknowledge on the prophet's mind.
Reply to Objection 2. Man is said to see in the First Truth the type of his existence, in so far as the image of the First Truth shines forth on man's mind, so that he is able to know himself.
Reply to Objection 3. From the very fact that future contingencies are in God according to unalterable truth, it follows that God can impress a like knowledge on the prophet's mind without the prophet seeing God in His essence.
Source: Summa Theologica (New Advent)