5 For us then, what are we? Already we are begotten of Him; but because we are such in hope, he says, “Beloved, now are we sons of God.” Now already? Then what is it we look for, if already we are sons of God? “And not yet,” says he, “is it manifested what we shall be.” But what else shall we be than sons of God? Hear what follows: “We know that, when He shall appear, we shall be like Him, because we shall see Him as He is.” Understand, my beloved. It is a great matter: “We know that, when He shall appear, we shall be like Him; for we shall see Him as He is.”
In the first place mark, what is called “Is.” You know what it is that is so called. That which is called “Is,” and not only is called but is so, is unchangeable: It ever remains, It cannot be changed, It is in no part corruptible: It has neither proficiency, for It is perfect; nor has deficiency, for It is eternal. And what is this? “In the beginning was the Word, and the Word was with God, and the Word was God.” And what is this? “Who being in the form of God, thought it not robbery to be equal with God.” To see Christ in this sort, Christ in the form of God, Word of God, Only-Begotten of the Father, equal with the Father, is to the bad impossible.
But in regard that the Word was made flesh, the bad also shall have power to see Him: because in the day of judgment the bad also will see Him; for He shall so come to judge, as He came to be judged. In the selfsame form, a man, but yet God: for “cursed is every one that puts his trust in man.” A man, He came to be judged, a man, He will come to judge. And if He shall not be seen, what is this that is written, “They shall look on Him whom they pierced?” For of the ungodly it is said, that they shall see and be confounded.
How shall the ungodly not see, when He shall set some on the right hand, others on the left? To those on the right hand He will say, “Come, you blessed of my Father, receive the kingdom:” to those on the left He will say, “Go into everlasting fire.” They will see but the form of a servant, the form of God they will not see. Why? Because they were ungodly; and the Lord Himself says, “Blessed are the pure in heart, for they shall see God.” Therefore, we are to see a certain vision, my brethren, “which neither eye has seen, nor ear has heard, nor has entered into the heart of man:” a certain vision, a vision surpassing all earthly beautifulness, of gold, of silver, of groves and fields; the beautifulness of sea and air, the beautifulness of sun and moon, the beautifulness of the stars, the beautifulness of angels: surpassing all things: because from it are all things beautiful.
Source: Homilies on the First Epistle of John (New Advent)