What does John 12:41 mean?

John has cited passages from the Old Testament prophet Isaiah. Among these are Isaiah 53, which speaks of a "Suffering Servant" and explains how people will reject Messiah and mistreat Him (John 12:38). He also referenced Isaiah 6:10, where God predicts how those who deliberately reject God will have their disobedience made permanent, as a form of judgment (John 12:40). Isaiah's prophecies are famous for featuring stunning images of God and His glory in heaven (Isaiah 6:1–7).

John's reference here is to how Isaiah "saw his glory and spoke of him." The "Him" in question here is Jesus—John is explicitly connecting the visions of the Old Testament prophet to Jesus of Nazareth. This fits with the main purpose of this gospel, which is to provide evidence that Jesus is, in fact, divine (John 20:30–31). The miracles and other acts by Jesus are all meant to support this idea (John 12:30), and those who reject those "signs" risk being left to their own stubbornness (John 12:37–40).