10 “Therefore the Jews did not believe concerning him, that he had been blind, and received his sight, till they called the parents of him that received his sight;” that is, who had been blind, and had come to the possession of sight. “And they asked them, saying, Is this your son, who ye say was born blind? How then does he now see? His parents answered them, and said, We know that this is our son, and that he was born blind: but how he now sees, we know not; or who has opened his eyes, we know not.
And they said, Ask himself; he is of age, let him speak of himself.” He is indeed our son, and we might justly be compelled to answer for him as an infant, because then he could not speak for himself: from of old he has had power of speech, only now he sees: we have been acquainted with him as blind from his birth, we know him as having speech from of old, only now do we see him endowed with sight: ask himself, that you may be instructed; why seek to calumniate us? “These words spoke his parents, because they feared the Jews: for the Jews had conspired already, that if any man did confess that He was Christ, he should be put out of the synagogue.”
It was no longer a bad thing to be put out of the synagogue. They cast out, but Christ received. “Therefore said his parents, He is of age, ask himself.”
Source: Tractates on the Gospel of John (New Advent)