Objection 1. It seems that one can in this life see the Divine essence. For Jacob said: "I have seen God face to face" (Genesis 32:30). But to see Him face to face is to see His essence, as appears from the words: "We see now in a glass and in a dark manner, but then face to face" (1 Corinthians 13:12).
Objection 2. Further, the Lord said to Moses: "I speak to him mouth to mouth, and plainly, and not by riddles and figures doth he see the Lord" (Numbers 12:8); but this is to see God in His essence. Therefore it is possible to see the essence of God in this life.
Objection 3. Further, that wherein we know all other things, and whereby we judge of other things, is known in itself to us. But even now we know all things in God; for Augustine says (Confess. viii): "If we both see that what you say is true, and we both see that what I say is true; where, I ask, do we see this? neither I in thee, nor thou in me; but both of us in the very incommutable truth itself above our minds." He also says (De Vera Relig. xxx) that, "We judge of all things according to the divine truth"; and (De Trin. xii) that, "it is the duty of reason to judge of these corporeal things according to the incorporeal and eternal ideas; which unless they were above the mind could not be incommutable." Therefore even in this life we see God Himself.
Objection 4. Further, according to Augustine (Gen. ad lit. xii, 24, 25), those things that are in the soul by their essence are seen by intellectual vision. But intellectual vision is of intelligible things, not by similitudes, but by their very essences, as he also says (Gen. ad lit. xiii, 24,25). Therefore since God is in our soul by His essence, it follows that He is seen by us in His essence.
On the contrary, It is written, "Man shall not see Me, and live" (Exodus 32:20), and a gloss upon this says, "In this mortal life God can be seen by certain images, but not by the likeness itself of His own nature."
I answer that, God cannot be seen in His essence by a mere human being, except he be separated from this mortal life. The reason is because, as was said above (Article 4), the mode of knowledge follows the mode of the nature of the knower. But our soul, as long as we live in this life, has its being in corporeal matter; hence naturally it knows only what has a form in matter, or what can be known by such a form. Now it is evident that the Divine essence cannot be known through the nature of material things. For it was shown above (Question 2, Article 9) that the knowledge of God by means of any created similitude is not the vision of His essence. Hence it is impossible for the soul of man in this life to see the essence of God. This can be seen in the fact that the more our soul is abstracted from corporeal things, the more it is capable of receiving abstract intelligible things. Hence in dreams and alienations of the bodily senses divine revelations and foresight of future events are perceived the more clearly. It is not possible, therefore, that the soul in this mortal life should be raised up to the supreme of intelligible objects, i.e. to the divine essence.
Reply to Objection 1. According to Dionysius (Coel. Hier. iv) a man is said in the Scriptures to see God in the sense that certain figures are formed in the senses or imagination, according to some similitude representing in part the divinity. So when Jacob says, "I have seen God face to face," this does not mean the Divine essence, but some figure representing God. And this is to be referred to some high mode of prophecy, so that God seems to speak, though in an imaginary vision; as will later be explained (II-II, 174) in treating of the degrees of prophecy. We may also say that Jacob spoke thus to designate some exalted intellectual contemplation, above the ordinary state.
Reply to Objection 2. As God works miracles in corporeal things, so also He does supernatural wonders above the common order, raising the minds of some living in the flesh beyond the use of sense, even up to the vision of His own essence; as Augustine says (Gen. ad lit. xii, 26,27,28) of Moses, the teacher of the Jews; and of Paul, the teacher of the Gentiles. This will be treated more fully in the question of rapture (II-II, 175).
Reply to Objection 3. All things are said to be seen in God and all things are judged in Him, because by the participation of His light, we know and judge all things; for the light of natural reason itself is a participation of the divine light; as likewise we are said to see and judge of sensible things in the sun, i.e., by the sun's light. Hence Augustine says (Soliloq. i, 8), "The lessons of instruction can only be seen as it were by their own sun," namely God. As therefore in order to see a sensible object, it is not necessary to see the substance of the sun, so in like manner to see any intelligible object, it is not necessary to see the essence of God.
Reply to Objection 4. Intellectual vision is of the things which are in the soul by their essence, as intelligible things are in the intellect. And thus God is in the souls of the blessed; not thus is He in our soul, but by presence, essence and power.
Source: Summa Theologica (New Advent)