What does John 9:29 mean?

Here, the religious leaders of Jerusalem continue to claim that they have a greater connection to truth than Jesus does. Their second attempt at interrogating a miraculously healed man is backfiring: the man has sarcastically pointed out their prejudice (John 9:27). In return, these educated men resort to insults and verbal abuse (John 9:28, 34).

Their claim not to know where Jesus comes from is further proof of stubbornness and dishonesty. Jesus' miracles are specifically meant to be "signs" of His divine power and approval (John 12:37–38). And yet, these men immediately reject the miracles because, to them, Jesus is a "sinner." They know the Scriptures of the Old Testament, but refuse to follow them to their intended conclusion: that Jesus is the Promised One (John 5:39–40). The only reason these man cannot identify Jesus' origin is pride: they do not know, or believe, because they do not want to know (John 7:17). Here, as in most cases, a person's intent is far more powerful than any evidence or reason others might provide.

The formerly-blind beggar's sarcastic answer has not only exposed their fragile egos, it has exposed their lack of reason. Things are about to get even worse for the scribes and Pharisees, however. The man they are insulting is about to strike back with an even more powerful point of logic.