D.A. Carson on John 12:41

Perhaps the most difficult statement in John 12 occurs in v. 41: Isaiah said this because he saw Jesus’ (lit. ‘his’, but the most natural antecedent is Jesus; but see below) glory and spoke about him. . .  It means that in his vision Isaiah saw (the pre-incarnate) Jesus. But there is a slightly different possibility. Targum Jonathan (an Aramaic paraphrase) to Isaiah 6:1 reads not ‘I saw the Lord’ but ‘I saw the glory of the Lord’, while the Targum to Is. 6;4 reads not ‘the King, the Lord of hosts’ but ‘the glory of the shekinah of the King of the ages, the Lord of hosts’. It may be necessary to appeal to the Targum; even in the Hebrew text Isaiah 6:3 already speaks of God’s glory. If instead we are to take the pronoun, as in the NIV, to refer to Jesus’ glory then John is unambiguously tying Jesus to Yahweh the Lord of hosts the Almighty—Isaiah saw Jesus in some pre-incarnate fashion . . .  What is remarkable, on this rendering of the passage, is the statement that Isaiah saw Jesus glory. This may be no more than the conclusion a chain of Christian reasoning: If the Son, the Word, was with God in the beginning, and was God, and if he was God’s agent of creation, and the perfect revelation of God to humankind, then it stands to reason that in those Old Testament passages where God is said to reveal himself rather spectacularly to someone, it must have been through the agency of his Son, his Word, however imperfectly the point was spelled out at the time. Therefore Isaiah said those words because (a stronger reading than ‘when’, AV) he saw Jesus’ glory.(D.A. Carson, The Gospel According to John [Nottingham, England: Apollos, 1991], 449-50)