God, Who is the Pure Act, with no mixture of potentiality, is, in Himself, supremely knowable, for the measure of cognition is Actuality; but because He so greatly exceeds our intellect He is not fully known by us; His Essence, however, can be seen, for our Beatitude consists in that Vision; were it otherwise, the desire implanted by nature would be futile if it could not attain to its First Cause, or would have to seek its Beatitude elsewhere than in the Vision of God. The Blessed do, therefore, see the Essence of God, although they do not comprehend Him.
The created intellect does not see the Divine Essence through any similitude created on the part of the Vision itself, which must be in some manner united with the seer, for the Essence of God is His Existence, and cannot be represented by any created form. It is rather uncircumscribed, containing in itself in a supereminent manner whatever can be known to, or understood by, the created intellect, and it is therefore beyond the capacity of representation by any created likeness: for every form is contained in a definition, and hence to say that God is seen through a similitude is equal to saying that the Divine Essence is not seen at all. We must rather say that the visual power is raised to beholding God through the Divine Light of Glory, according to the words, "In Thy Light we shall see Light."
God, being incorporeal, is not seen by the bodily eye; but those in the flesh after the resurrection will see Him in such a manner as not by any sense or faculty of the senses, but only by the intellect; and in vision of the imagination will be a kind of likeness of God. Nor can the Divine Essence be seen through the natural power of any created intellect, because, as knowledge comes by mode, and the nature of the one that knows, and in such a manner that another superior faculty is required to attain to any object above nature, and God being His own essential Existence, can, therefore, only be seen by the created intellect through Grace. The created intellect is in need of some disposition of a supernatural order to see the Essence of God; that Divine Essence is, indeed, the Form which determines our intellect; hence it is necessary that by Divine Grace a power of intelligence should be added to nature, and this increase of intellectual strength is that illumination of the intellect of which we read (Apocalypse 21), and it is said that the Glory of God shall enlighten the society of the Blessed who see God. God Himself shall be seen more perfectly by one than by another, accordingly as one shall participate in Glory and Light more than another, and he shall participate more who has more of charity, for where there is more charity there is more desire, and desire makes him who desires in some manner apt and prepared for the reception of the object desired. Inasmuch, however, as God is capable of being known to infinity, He cannot be comprehended by the created intellect, although He is so in a certain sense, inasmuch as the Blessed see His Essence, and seeing they possess, and possessing they enjoy.
Although the Divine Essence is seen by the created intellect, everything in God is not seen, otherwise He would be fully understood, which, as we have seen, is impossible. The natural desire of the creature, however, will be satisfied by the understanding of genus and species with their causes, all of which will be seen in this same Essence, and the higher the intellect the more will be known. The perfection of the created intellect does not require the individual knowledge of things with all their details, nor do we naturally desire this: God alone, the Source and Principle of all Being, fully satisfies all.
Those who see God will not see anything else by its own likeness, but by the Divine Essence itself united to their intellect; and they will see all things at once in God, and not successively, for as in this present life our intellect understands by many and various species, and it cannot by one act be filled with many species, as one body cannot be made of several different shapes at once, the intellect of the Blessed sees all things by the one Divine Essence and not by any likeness: and, therefore, it sees all things at once, and not successively. In this life no one can see the Divine Essence, because whilst we are in the body we understand by the mode that is natural to us, and our soul in this life exists in corporeal matter; whereas the Divine Essence cannot be known through the nature of corporeal things. The Vision of God by means of some created similitude is not the Vision of the Divine Essence Itself. If Moses and Saint Paul, the one ruler of the Jews, the other of the Gentiles, are objected as having been raised above the course of nature to see the Divine Essence, it is explained that neither of them were at that moment subject to the senses. Likewise by natural reason we cannot know the Divine Essence; but by things of sense we can learn that God exists, and that He is the Cause of all things, but in Himself He is nothing of those things of which He is the Cause, and which are removed from Him, not by way of His being deprived of anything, but because He far exceeds them all. We have by Grace a higher knowledge of Divine things than reason can give, for when the natural light of the intellect is strengthened by Divine Light, images more expressive of Divine realities are produced than any within reach of nature. This is evident from the prophetic visions; also when voices or other sensible things are supernaturally produced in order to convey some experience of Divine things, as when the Holy Ghost, at Christ's baptism, appeared in the form of a dove, and the Father's voice was heard: "This is My Son," etc.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni