“Isaiah said this
Because he saw his glory.
He spoke about him.”
ταῦτα εἶπεν Ἡσαΐας ὅτι εἶδεν τὴν δόξαν αὐτοῦ, καὶ ἐλάλησεν περὶ αὐτοῦ.
Then John uniquely indicated that Isaiah (Ἡσαΐας) had said this (ταῦτα εἶπεν) because he had seen (ὅτι εἶδεν) the glory of Yahweh, God (τὴν δόξαν αὐτοῦ). He had spoken (καὶ ἐλάλησεν) about him (περὶ αὐτοῦ), God the Father. John explained that Isaiah had seen and spoken of the glory of Yahweh, God, the Father. Thus, Isaiah had predicted the coming of Jesus as being sent from the Father. The Christological theology of John is very clear with this glorification of the Son that came from the glorified Father. How do you glorify Jesus and the Father?