John 12:41

In approaching the account of his coming to humanity, the prophecy before us tells first of his divine kingdom, in which it says that the prophet saw him sitting on a throne high and exalted. This is that throne that is mentioned in the psalm of the Beloved. … John the Evangelist supports my interpretation of this passage, when he quotes the words of Isaiah, where it is said, “For this people’s heart has become fat, and their ears are dull of hearing, and they have closed their eyes,” referring them to Christ, saying, “This is what Isaiah said when he saw his glory and bore witness of him.” The prophet then seeing our Savior sitting on his Father’s throne in the divine and glorious kingdom, and moved by the Holy Spirit and being about to describe next his coming among humanity and his birth of a Virgin, foretells that his knowledge and praise would be over all the earth.

(Hom. lxviii. 2) His glory means the vision of Him sitting on His lofty throne: I saw the Lord sitting upon a throne. Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? (Is. 6:1)

(Tr. liii. 11) These things said Esaias, when he saw His glory, and spake of Him. He saw Him not really, but figuratively, in prophetic vision. Be not deceived by those who say that the Father is invisible, the Son visible, making the Son a creature. For in the form of God, in which He is equal to the Father, the Son also is invisible; though He took upon Him the form of a servant, that He might be seen by men. Before His incarnation too, He made Himself visible at times to human eyes; but visible through the medium of created matter, not visible as He is.

"These things said Isaiah, when he saw His glory, and spake of Him." What Isaiah saw, and how it refers to Christ the Lord, are to be read and learned in his book. For he saw Him, not as He is, but in some symbolical way to suit the form that the vision of the prophet had itself to assume. For Moses likewise saw Him, and yet we find him saying to Him whom he saw, "If I have found grace in Thy sight, show me now Thyself, that I may clearly see Thee;" for he saw Him not as He is. But the time when this shall yet be our experience, that same Saint John the Evangelist tells us in his Epistle: "Dearly beloved, [now] are we the sons of God; and it hath not yet become manifest what we shall be: because we know that, when He shall appear, we shall be like Him; for we shall see Him as He is." He might have said "for we shall see Him," without adding "as He is;" but because he knew that He was seen of some of the fathers and prophets, but not as He is, therefore after saying "we shall see Him," he added "as He is." And be not deceived, brethren, by any of those who assert that the Father is invisible, and the Son visible. This assertion is made by those who think that the latter is a creature, and whose understanding runs not in harmony with the words, "I and my Father one." Accordingly, as respects the form of God wherein He is equal with the Father, the Son also is invisible: but, in order to be seen of men, He assumed the form of a servant, and being made in the likeness of men, became visible to man. He showed Himself, therefore, even before His incarnation, to the eyes of men, as it pleased Him, in the creature-form at His command, but not as He is. Let us be purifying our hearts by faith, that we may be prepared for that ineffable and, so to speak, invisible vision. For "blessed are the pure in heart; for they shall see God."

"These things said Isaiah, when he saw His glory." Whose? The Son's. Although the prophet, judging by the connection of the speech, seems to have seen the glory of the Father, the evangelist says here that Isaiah saw the glory of the Son, and the apostle Paul says that he saw the glory of the Spirit (Acts 28:25). Truly the glory of the Holy Trinity—Father, Son, and Holy Spirit—is one.
"Isaiah," He says, "saw His glory": the smoke that appeared to him, the seraphim, the coals, the altar, the throne (Isa. 6:1–7). So Isaiah saw this glory and spoke of Him, that is, of the Son. What did he say of Him? That which was said above, that He blinded their eyes and hardened their hearts.

Then the Evangelist shows that these words of Isaiah apply here. He says, Isaiah said this because he saw his glory, the glory of God. For when he saw the glory of God he saw at the same time that the Jews would be blinded, as is clear from, "I saw the Lord seated on a high and lofty throne" (Is 6:1), followed by, "Blind the heart of this people and make their ears heavy, and shut their eyes, lest they see with their eyes and hear with their ears and understand with their heart and be converted, and I heal them" (Is 6:10). And because it is fitting that one should testify about what he has seen - as we read in 1 John (1:1) - he adds, and spoke of him, that is, of Christ, whose glory he saw: "To him all the prophets bear witness" (Acts 10:43); "Which he promised beforehand through his prophets in the Holy Scriptures, the gospel concerning his Son" (Rom 1:2).

We read that Isaiah saw and said these things. As to the first, we should avoid the error of the Arians, who say that the Father alone is invisible to every creature, but that the Son was seen in the visions of the Old Testament. But since it is stated that "He who has seen me has seen the Father" (14:9), it is obvious that the Father and the Son are visible in one and the same way. And so Isaiah, seeing the glory of the Son, also saw the glory of the Father, and indeed of the entire Trinity, which is one God, seated upon a high throne before whom the seraphim cry out: Holy, Holy, Holy! This does not mean that Isaiah saw the essence of the Trinity; rather in an imaginary vision, with understanding, he expressed certain signs of this majesty, according to the saying in Numbers (12:6): "If there is a prophet among you, I the Lord make myself known to him in a vision, I speak with him in a dream."

As to the second thing, that Isaiah spoke of him: this excludes the error of the Manicheans, who said that there were no prophecies about Christ in the Old Testament, as Augustine reports to us in his book Against Faustus; and it excludes the error of Theodore of Mopsuestia, who said that all the prophecies of the Old Testament bore on some current event, but the apostles and evangelists appropriated them to the life of Christ, like things said about one event can be appropriated to another event. But all this is excluded by the statement, and spoke of him, just as Christ said of Moses that "he wrote of me" (5:46).