These things spoke Jesus. Lest readers should think that this prediction was inappropriately quoted, John expressly states, that the prophet was not sent as a teacher to a single age, but, on the contrary, that the glory of Christ was exhibited to him, that he might be a witness of those things which should take place under his reign. Now the Evangelist takes for granted, that Isaiah saw the glory of Christ; and hence he infers, that Isaiah accommodates his instruction to the future state of Christ's kingdom.

When he saw his glory - Isaiah 6:1-10. Isaiah saw the Lord (in Hebrew, יהוה Yahweh) sitting on a throne and surrounded with the seraphim. This is perhaps the only instance in the Bible in which Yahweh is said to have been seen by man, and for this the Jews affirm that Isaiah was put to death. God had said Exodus 33:20, "No man shall see me and live;" and as Isaiah affirmed that he had seen Yahweh, the Jews, for that and other reasons, put him to death by sawing him asunder. See Introduction to Isaiah, Section 2. In the prophecy Isaiah is said expressly to have seen Yahweh John 12:1; and in John 12:5, "Mine eyes have seen the King Yahweh of hosts." By his glory is meant the manifestation of him - the Shechinah, or visible cloud that was a representation of God, and that rested over the mercy-seat. This was regarded as equivalent to seeing God, and John here expressly applies this to the Lord Jesus Christ; for he is nor affirming that the people did not believe in God, but is assigning the reason why they believed not on Jesus Christ as the Messiah. The whole discourse has respect to the Lord Jesus, and the natural construction of the passage requires us to refer it to him. John affirms that it was the glory of the Messiah that Isaiah saw, and yet Isaiah affirms that it was Yahweh; and from this the inference is irresistible that John regarded Jesus as the Yahweh whom Isaiah saw. The name Yahweh is never, in the Scriptures, applied to a man, or an angel, or to any creature. It is the unique, incommunicable name of God. So great was the reverence of the Jews for that name that they would not even pronounce it. This passage is therefore conclusive proof that Christ is equal with the Father.
Spake of him - Of the Messiah. The connection requires this interpretation.

When he saw his glory - Isaiah 6:1, etc. I saw Jehovah, said the prophet, sitting upon a throne, high and lifted up, and his train filled the temple. Above it stood the seraphim; and one cried unto another, and said, Holy, holy, holy, is Jehovah, God of hosts; the whole earth shall be full of his glory! It appears evident, from this passage, that the glory which the prophet saw was the glory of Jehovah: John, therefore, saying here that it was the glory of Jesus, shows that he considered Jesus to be Jehovah. See Bishop Pearce. Two MSS. and a few versions have Θεου, and του Θεου αὑτου, the glory of God, or of his God.

These things said Esaias,.... Concerning the blinding and hardening of the Jews:
when he saw his glory, and spake of him; when he saw, in a visionary way, the glory of the Messiah in the temple, and the angels covering their faces with their wings at the sight of him; and when he spake of him as the King, the Lord of hosts, whom he had seen, Isaiah 6:1, from whence it is clear that he had respect to the Jews in the times of the Messiah. The prophet says in Isaiah 6:1 that he "saw the Lord": the Targumist renders it, "I saw", , "the glory of Jehovah"; and in Isaiah 6:5 he says, "mine eyes have seen the King", Jehovah, Zebaot, the Lord of hosts; which the Chaldee paraphrase renders, "mine eyes have seen", , "the glory" of the Shekinah, the King of the world, the Lord of hosts. Agreeably to which our Lord says here, that he saw his glory, the glory of his majesty, the glory of his divine nature, the train of his divine perfections, filling the temple of the human nature; and he spoke of him as the true Jehovah, the Lord of hosts; and which therefore is a very clear and strong proof of the proper divinity of Christ. And it may be observed from hence, that such persons who have a true, spiritual, and saving sight of Christ, of the glory of his person, and the fulness of his grace, cannot but be speaking of him to others, either in private, or in public, as Isaiah here did, and as the church in Song 5:10; and as the apostles of Christ, John 1:1; and indeed, should they hold their peace, the stones would cry out; such must, and will speak of his glory in his temple, Psalm 29:9.

These things said Esaias, when he saw his glory, and spake of him--a key of immense importance to the opening of Isaiah's vision (Isaiah 6:1-13), and all similar Old Testament representations. "THE SON is the King Jehovah who rules in the Old Testament and appears to the elect, as in the New Testament THE SPIRIT, the invisible Minister of the Son, is the Director of the Church and the Revealer in the sanctuary of the heart" [OLSHAUSEN].

When he saw his glory - Christ's, Isaiah 6:1, &c. And it is there expressly said to be the glory of the Lord, Jehovah, the Supreme God.