What does John 12:40 mean?
This is a reference to Isaiah 6:10, which John phrases in the present tense, indicating it's being fulfilled. God is "blinding" and "hardening" the people so that they will not accept the truth. Taken entirely out of context, that would seem to be spiteful and unfair. How can God blame people for disbelief (John 3:36) when He is purposefully blocking them from it?The answer is in the full context of this passage. Just recently, John described a crowd speaking to Jesus (John 12:20–22), who heard literal words from heaven (John 12:28). Many in the crowd dismissed that as thunder (John 12:29). Religious experts attacked Jesus, even though their own studies ought to have led them to the truth (John 5:39–40). In the same way, many people ignore clear evidence of God (Romans 1:18–20), even in nature (Psalm 19:1), because they do not want to believe. No miracle will ever convince those who are committed to defying God (Luke 16:31).
This means that God can, and does, use "hardening" and "blinding" as a form of judgment—after a person has already committed themselves to rejecting Him. In Exodus, God speaks of how He will "harden the heart" of the Egyptian king to prolong the plagues demonstrating God's glory (Exodus 4:21). Eventually, God does just that (Exodus 9:12). But God doesn't influence Pharaoh until Pharaoh has repeatedly and frequently hardened his own heart (Exodus 7:13 22; 8:15, 19, 32).
John cites this Scripture as proof that those who stubbornly disbelieve in God have only themselves to blame. At some point, even their inability to accept God is a consequence of their own rebellion.