What does John 12:37 mean?

This verse echoes a theme many people resist: unbelief is mostly about what a person wants, not what a person knows. Jesus lambasted His religious critics for "refusing" to acknowledge the obvious conclusion of their own study (John 5:39–40). He pointed out that even seeing a miracle wouldn't convince those who ignored prior evidence (Luke 16:31). That message was confirmed when Jesus raised a man from death, and His enemies responded by plotting to assassinate both the miracle-worker and the restored man (John 11:53; 12:9–11)! In the prior conversation, Jesus was affirmed by an actual voice, from heaven, speaking audible words (John 12:28). Some of the people who heard it dismissed it as thunder (John 12:29).

In that same encounter, Jesus pointed out that His miracles—and other forms of testimony—were "for" the people (John 12:3). They were given by God specifically to point towards belief in Christ. Any person who claims they need "more evidence" to believe in Jesus is lying—either to themselves, or outright, to others. The proof we're given in nature (Psalm 19:1), experience (Romans 1:18–20), Scripture (John 20:30–31) and human testimony (Matthew 28:19) is more than enough. What stops people from responding to the gospel is not lack of knowledge. It's sinful stubbornness.

Some interpreters point to the following verses and claim that disbelief is not "their fault," since God is credited with blinding eyes and hardening hearts (John 12:39–40). As is often said, the problem is simply that people "could not" believe because they "would not" believe. This concept is connected by the upcoming reference to Isaiah (John 12:38–39). Scripture indicates that God can respond to willful stubbornness by "cutting off" a person (Proverbs 29:1). Those who resist God might find He hardens their hearts as judgment on their non-belief (Exodus 9:12).