When questioned about their son's healing, the man's parents confirm that he is their son and that he was born blind, but they refuse to say how he now sees. John explains their caution: they fear the authorities because anyone confessing Jesus as Messiah could be put out of the synagogue.
The threat gives the interrogation a social consequence beyond theological disagreement.
The Greek adjective aposynagogos means excluded from the synagogue or synagogue community. Exactly what procedures and duration such exclusion involved in John's historical setting are debated, and later rabbinic disciplinary formulas should not automatically be read back into the Gospel. Nevertheless, synagogue membership involved worship, communal identity, relationships, and social standing.
The parents therefore protect themselves by insisting that their adult son answer for himself.
The context explains why the parents' caution should not simply be caricatured as indifference to their son. Public confession could carry meaningful communal costs.
John uses that fear as a contrast with the healed man, whose testimony grows increasingly bold until he is cast out and then encounters Jesus again.