After quoting Isaiah 6, John makes an extraordinary interpretive statement: 'These things said Esaias, when he saw his glory, and spake of him.' In context, the pronouns point to Jesus.
John therefore connects Isaiah's vision with the glory of Christ.
Isaiah 6 describes the prophet seeing the Lord enthroned in overwhelming holiness while seraphim proclaim, 'Holy, holy, holy.' John has just used that chapter to explain unbelief and then says Isaiah saw 'his glory' and spoke of Jesus. The statement should be read within John's larger Christology, beginning with the Word who was with God and was God and through whom all things were made.
John does not merely say that Isaiah predicted a later teacher; he identifies the glory seen by Isaiah with Jesus' glory.
The Old Testament setting gives the verse exceptional weight. Divine glory in Isaiah's throne vision is not casual honor bestowed on an ordinary human figure.
John's identification places Jesus within the divine identity while preserving the Gospel's distinction between the Son and the Father. It is a major bridge between Israel's Scriptures and John's presentation of Christ.