What does John 9:18 mean?

Jesus has miraculously given eyesight to a man born blind (John 9:1–7). This fulfills prophecies of the Messiah (Isaiah 29:18; 35:5; 42:7). It also occurred on a Sabbath day (John 9:14), further irritating the religious leaders of Jerusalem.

The gospel of John usually uses the term "the Jews" as a reference to the spiritual and religious leaders of the nation of Israel and those who follow their leadership. Often, that specifically means scribes and Pharisees. Here, as in other instances, Jesus' harshest critics and greatest resistance come from the very people who ought to know better.

The fact that this man had always been blind, and was known to be blind, leads to extreme skepticism. The religious leaders of Jerusalem simply refuse to believe this man has been given sight. They summon his parents. Their hope is that they can disprove that this is the same man known as a blind-from-birth beggar. The skepticism of these leaders is not entirely unjustified. Their role is to shepherd the people of Israel. That includes investigating spiritual claims. Where they go wrong is not in asking questions. Rather, the error is in taking cynicism to such extreme levels that they refuse to believe, no matter what.