Objection 1. It seems that the essence of God can be seen by the corporeal eye. For it is written (Job 19:26): "In my flesh I shall see... God," and (Job 42:5), "With the hearing of the ear I have heard Thee, but now my eye seeth Thee."
Objection 2. Further, Augustine says (De Civ. Dei xxix, 29): "Those eyes" (namely the glorified) "will therefore have a greater power of sight, not so much to see more keenly, as some report of the sight of serpents or of eagles (for whatever acuteness of vision is possessed by these creatures, they can see only corporeal things) but to see even incorporeal things." Now whoever can see incorporeal things, can be raised up to see God. Therefore the glorified eye can see God.
Objection 3. Further, God can be seen by man through a vision of the imagination. For it is written: "I saw the Lord sitting upon a throne," etc. (Isaiah 6:1). But an imaginary vision originates from sense; for the imagination is moved by sense to act. Therefore God can be seen by a vision of sense.
On the contrary, Augustine says (De Vid. Deum, Ep. cxlvii): "No one has ever seen God either in this life, as He is, nor in the angelic life, as visible things are seen by corporeal vision."
I answer that, It is impossible for God to be seen by the sense of sight, or by any other sense, or faculty of the sensitive power. For every such kind of power is the act of a corporeal organ, as will be shown later (78). Now act is proportional to the nature which possesses it. Hence no power of that kind can go beyond corporeal things. For God is incorporeal, as was shown above (Question 3, Article 1). Hence He cannot be seen by the sense or the imagination, but only by the intellect.
Reply to Objection 1. The words, "In my flesh I shall see God my Saviour," do not mean that God will be seen with the eye of the flesh, but that man existing in the flesh after the resurrection will see God. Likewise the words, "Now my eye seeth Thee," are to be understood of the mind's eye, as the Apostle says: "May He give unto you the spirit of wisdom... in the knowledge of Him, that the eyes of your heart" may be "enlightened" (Ephesians 1:17-18).
Reply to Objection 2. Augustine speaks as one inquiring, and conditionally. This appears from what he says previously: "Therefore they will have an altogether different power (viz. the glorified eyes), if they shall see that incorporeal nature;" and afterwards he explains this, saying: "It is very credible, that we shall so see the mundane bodies of the new heaven and the new earth, as to see most clearly God everywhere present, governing all corporeal things, not as we now see the invisible things of God as understood by what is made; but as when we see men among whom we live, living and exercising the functions of human life, we do not believe they live, but see it." Hence it is evident how the glorified eyes will see God, as now our eyes see the life of another. But life is not seen with the corporeal eye, as a thing in itself visible, but as the indirect object of the sense; which indeed is not known by sense, but at once, together with sense, by some other cognitive power. But that the divine presence is known by the intellect immediately on the sight of, and through, corporeal things, happens from two causes--viz. from the perspicuity of the intellect, and from the refulgence of the divine glory infused into the body after its renovation.
Reply to Objection 3. The essence of God is not seen in a vision of the imagination; but the imagination receives some form representing God according to some mode of similitude; as in the divine Scripture divine things are metaphorically described by means of sensible things.
Source: Summa Theologica (New Advent)