His parents said this because they were afraid of the Jews. For the Jews had already determined that anyone who confessed Jesus as the Christ would be put out of the synagogue.
22. The Jews had determined. This passage shows that the custom of excommunication is ancient, and has been observed in all ages; for excommunication was not then for the first time invented, but it was a custom which had been anciently used against apostates and despisers of the Law, and was turned against the disciples of Christ.
One would have expected that such a miracle as Christ wrought upon the blind man would have settled his reputation, and silenced and shamed all opposition, but it had the contrary effect; instead of being embraced as a prophet for it, he is prosecuted as a criminal. I.
Commenting on John 9:13-34
These words spake his parents,.... these were the answers they returned to the three questions put to them: and the reason why they answered in the manner they did to the third, was, because they feared the Jews; the Jewish sanhedrim, otherwise they were Jews themselves: for the Jews had agreed already; the sanhedrim had made a decree, either at this time, upon this account...