In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple.
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Cyril of Alexandria
No one can deny that the prophet saw the Son in the glory of God the Father, as John said: “Isaiah said this because he saw his glory and spoke of him [Christ].” Look and see the great honor that is due to God, and see the authority he has over all creation. God is high and lifted up on a throne, crowned with the splendor of his reign.… In my view we should not think of the throne of God as lifted up in a physical way. That would be foolish and absurd. Rather, that the throne is said to be lifted up means that the reign of God transcends all things. That God is sitting refers to his immovability and that his blessings are everlasting and unchanging. - "Commentary on Isaiah 1.4"
Died. Either a natural (Calmet) or a civil death, by means of the leprosy. (Chaldean) (To stat. 7.)
This and the former chapters relate to the commencement of Joathan's reign, whether before or after the death of Ozias. (Calmet)
Many think that this was the first prediction of Isaias. (Origen) (St. Jerome, ad Dam.)
I saw. By a prophetic vision, as if I had been present at the dedication of the temple, 3 Kings viii. 10. (Calmet)
Lord. Not the Father, as some have asserted, but the Son, John xii. 40. (St. Jerome, ad Dam.) (Calmet)
Neither Moses nor any other saw the substance of God; but only a shadow. Yet Manasses hence took a pretext to have Isaias slain. (Origen) (St. Jerome, Trad.) (Paralipomenon) (Worthington)
We have talked about standing; we have talked about walking; let us talk about sitting. Whenever God is represented as seated, the portrayal takes one of two forms: either he appears as the ruler or as the judge. If he is like a king, one sees him as Isaiah does: “I saw the Lord seated on a high and lofty throne.” There he is presented as the sovereign king. - "Homilies on the Psalms 14 (Psalm 81)"
It is obvious from the very words of Isaiah that he saw God because of God’s condescension. He said, “I saw the Lord sitting on a high and lofty throne.” But God is not sitting down. Beings with bodies sit. Isaiah also said, “on a throne.” But God is not encompassed by a throne, because divinity cannot be contained within boundaries. That said, the seraphim could not endure the condescension of God although they were nearby.… He said, “And the seraphim stood around him,” because he wanted to make it clear that although the seraphim are closer to the essence of God than human beings are, they cannot look upon his essence simply because they are closer to it. He is not referring to place in a localized sense. When he speaks of nearness, he is demonstrating that the seraphim are closer to God than we human beings are. - "Against the Anomoeans 3.16"