What does John 9:21 mean?

After Jesus gives sight to man born blind (John 9:1–7), controversy erupts. As with many of Jesus' miracles, the healing occurred on a Sabbath day (John 9:14) in violation of traditional rules. Local religious leaders, referred to as "the Jews" in the gospel of John, have made it clear that anyone who supports Jesus is subject to excommunication (John 9:22). This would mean being cut off from nearly all community life. Seeking to debunk Jesus' miracle, the scribes and Pharisees summoned the parents of the formerly blind man. They ask if he was really their son, and really born blind. The parents have already testified to both.

Here, the parents respond to other questions raised by Jesus' miraculous act. scribes and Pharisees repeatedly ask "how" Jesus has accomplished this feat. This shows their inherent prejudice: they cannot accept that Jesus is healing with divine power. So, they seek some other explanation. The meaning of the miracle—the message behind the "sign"—is lost on them since they refuse to believe in Jesus and His words (John 5:39–40).

The parents pointedly state that they don't know "who" healed their son. While this is almost certainly true, it might also be their strategy to avoid trouble. Those who identify Jesus as the Messiah were subject to virtual exile from the community (John 9:22). The healed man's parents are extremely careful not to cross that line. The parents also point out that the man in question is a legal adult; he can answer these questions for himself, so there is no reason to require testimony from his parents.