‘The Judaisers therefore did not believe about him that he had been blind and had received his sight until they called the parents of him who had received his sight.'
But they were not prepared to believe his words. Note the change from ‘the Pharisees' to ‘the Judaisers'. Not all Pharisees were antagonistic to Jesus as we have seen, and they were prepared to wait and see. ‘The Judaisers', which is John's term here for those who were antagonistic, were not willing to be convinced and did not believe the man's story. So they called in the man's parents. This may have been the beginning of an official enquiry, or just a preliminary vetting.
It was quite an awe inspiring thing for these people to be brought before a gathering of the leading religious authorities. They knew that such men could have them excluded from the privileges of the synagogue.