What does John 9:24 mean?

This is the second time local religious leaders have questioned a man whom Jesus healed of lifelong blindness (John 9:1–7). The man's initial claims were brushed off as impossible; scribes and Pharisees had already decided Jesus was a fraud (John 5:39–40). The investigators then turned to the man's parents, hoping to debunk some part of the story. Instead, they confirmed that this man had, in fact, been born blind. The parents, fearing backlash from the religious leaders (John 9:22), had said very little and reminded everyone that their son was old enough to speak for himself.

The command "give glory to God" here is a way of demanding that a person speak the truth. Using it typically implies the other person needs to confess some sin or deception (Joshua 7:19; 1 Samuel 6:5). This is somewhat like a modern-day judge telling a defendant, "come, now, tell the truth…" The religious leaders further prove their prejudice by stating "we know…" Jesus is a sinner. This echoes their initial rejection of the miracle: they refused to accept it because they had already decided that Jesus didn't agree with their traditions (John 9:16). By hinting that the man is lying, and claiming to "know" that Jesus is false, the scribes and Pharisees are trying to intimidate the formerly-blind man into changing his story.

This effort will backfire spectacularly. In the following verses, the healed man will produce a profound summary of saving faith. His simple, common-sense approach to facts will embarrass the hard-headed, willfully ignorant religious leaders.