John 12:41

When he saw his glory - Isaiah 6:1. 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.