Proposition # 20: John 12:41 says that Isaiah saw Jesus’ glory and spoke of him. This refers back to Isaiah 6 which opens, “In the year that King Uzziah died I saw the Lord sitting upon a throne, high and lifted up; and his train filled the temple.” And in verse 5 Isaiah says, “My eyes have seen the King, Yahweh of hosts.” This proves that Jesus is God.
Response: Since one cannot literally see God, this sighting must have been in a non-literal sense. (John 1:18) Upon seeing God Isaiah was moved to dread as he was of “unclean lips.” One of the seraphim cleansed Isaiah’s lips with a burning coal from the altar. Then God inquires, “Whom shall I send, and who shall go for Us?” Isaiah, having been cleansed, is qualified to respond to God’s call. The remaining verses in Isaiah 6 speak of the response to Isaiah’s ministry, and, as John shows, prophetically to Jesus’ ministry; and the response in both cases was primarily one of rejection. — Isaiah 6:1-8
What does this have to do with Isaiah’s seeing Jesus’ glory? The rejection of Jesus led to his crucifixion which was his pathway to glorification. Recall what Jesus said to the two disciples on the road to Emmaus, “Was it not necessary for the Messiah to suffer these things and to enter into his glory?” In prophetic vision Isaiah foresaw Christ’s future: his rejection, his crucifixion, and his glorification. – Luke 24:26; consider John 12:16, John 12:23, John 12:24; John 13:31-32; and John 17:1