‘Therefore they could not believe, for Isaiah again said, “He has blinded their eyes and hardened their heart, lest they should see with their eyes and perceive with their heart, and turn for me to heal them”. Isaiah said this because he saw his glory and spoke of him.'
When does God blind men's eyes and harden men's hearts? It is when they have first closed their own eyes and deliberately hardened their own hearts. Compare the example of Pharaoh in Exodus. He first hardened his own heart, and then later on it was God Who hardened his heart. Thus having refused to respond such people become in danger of permanent blindness, for God's actions from then on will only further blind and harden them. It is dangerous to play with God's truth.
What a contrast there was between Isaiah and the people. Isaiah opened his eyes and saw the glory of God when God revealed His glory. (John may well have had in mind that in the same way Jesus was the revealer of the glory of God (John 1:14)). But the people closed their eyes to that revelation of God. The people here too were closing their eyes in the light of this new revelation of the glory of God (‘we beheld His glory' - John 1:14) and may also become blinded by God. This was what those who had seen Jesus' signs and failed to respond were in danger of doing.
‘Isaiah said this because he saw his glory and spoke of him.' In context the pronouns here refer to Jesus. The writer is thus stating that when Isaiah saw the glory of the Lord, it was the glory of the Lord Jesus that he saw. To him the Lord God and the Lord Jesus were inextricably linked. Once again we have the emphasis that Jesus is the ‘true Son of God'.