Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 187-188.
This verse [i.e. John 9:22] is significant from a socio-historical perspective because we know that there was no official policy against accepting Jesus (or anyone else) as messiah during his lifetime. On the other hand, some Jewish synagogues evidently did begin to exclude members who believed in Jesus’ messiahship toward the end of the first century. So the story of Jesus healing the blind man reflects the experience of the later community that stood behind the Fourth Gospel. These believers in Jesus had been expelled from the Jewish community, the community, presumably, of their families and friends and neighbors, in which they had worshiped God and had fellowship with one another.
Their expulsion from their synagogue had serious implications for the Christian community’s social life and for the way it began to understand its world and its stories about its messiah, Jesus.