"tn Grk “his”; the referent (Christ) has been specified in the translation for clarity. The referent supplied here is “Christ” rather than “Jesus” because it involves what Isaiah saw. It is clear that the author presents Isaiah as having seen the preincarnate glory of Christ, which was the very revelation of the Father (see John 1:18; John 14:9). sn Because he saw Christ’s glory. The glory which Isaiah saw in Isa 6:3 was the glory of Yahweh (typically rendered as “Lord” in the OT). Here John speaks of the prophet seeing the glory of Christ since in the next clause and spoke about him, “him” can hardly refer to Yahweh, but must refer to Christ. On the basis of statements like 1:14 in the prologue, the author probably put no great distinction between the two. Since the author presents Jesus as fully God (cf. John 1:1), it presents no problem to him to take words originally spoken by Isaiah of Yahweh himself and apply them to Jesus."