The Bar Kokhba Revolt
See Ancient Jewish History: The Bar-Kokhba Revolt for a brief discussion of the Jewish and Christian positions leading up to the revolt.
See Lawrence H. Schiffman https://www.biblicalarchaeology.org/daily/ancient-cultures/daily-life-and-practice/the-jewish-christian-schism/ from which the following is extracted.
132 CE – During the Bar Kokhba Revolt, the Christians, unable to support the Messianic pretensions of Bar Kokhba, sided with the Romans. By the end of the Bar Kokhba revolt, the Rabbis regarded the entire Christian community as non-Jewish. Even the Bishop of Jerusalem was now gentile since Jews (even Jewish Christians) were prohibited from living in the Holy City. It no longer mattered that a few of the Christians were technically Jewish. The lack of Jewish status of the group as a whole led the Rabbis to disqualify them as a whole. Henceforth, from the Rabbinic perspective, the Christians were a separate religion and a separate people. Marriage with them was now prohibited.
It is also possible to follow this process of separation from the perspectives of the Romans and the Christians. The Romans were outsiders who concluded that the schism was permanent at the end of the first century and, accordingly, began to regard the Christians as a separate religious community, excused from paying the fiscus Judaicus, the Jewish poll tax.