The Pharisees, unable to extract damaging testimony from the parents, begin to interrogate the healed man more thoroughly. Apparently, he had been absent during the questioning of his parents, because the Pharisees attempt to finesse an admission out of him by pretending that they had learned the true story from them (John 9:24). In the ensuing exchange, they amplify their position (John 9:29).
The Pharisees also claim both to know and not to know something. They claim to know that Jesus is a sinner and that God had spoken through Moses. They claim not to know Christ's origin. Yet, what they claim and what they deny contradict (John 9:29; 7:27)—they are lying! Unlike the parents, who know the truth but will not admit it, these men think they know the truth but are actually ignorant of it.
Sadly, this also describes many people today, particularly those pseudo-scholars and pseudo-leaders who claim to know all about Christ and Christianity but have never really come to know Him personally.
Martin G. Collins
The Miracles of Jesus Christ: Healing a Man Born Blind (Part Three)