Albert Barnes Commentary John 9:34

"They answered and said unto him, Thou wast altogether born in sins, and dost thou teach us? And they cast him out." — John 9:34 (ASV)

Wast born in sins. That is, you were born in a state of blindness—a state which proved that either you or your parents had sinned, and that this was the punishment for it. See John 9:2. You were cursed by God with blindness for crime, and yet you set yourself up as a religious teacher!

When people have no arguments, they attempt to substitute them with revilings. When pressed by argument, they reproach their adversaries with accusations of crime, and especially with being blind, perverse, heretical, disposed to speculation, and regardless of the authority of God. And they especially consider it great presumption when someone of a lower age or rank presumes to advance an argument against prevailing opinions.

They cast him out. Out of the synagogue. They excommunicated him. See the notes on John 9:22.