Rabbi Sabbatai Zevi, the Sabbatean messiah, was forced to convert to Islam by the Ottoman Grand Vizier under threat of gruesome death. What is less known is that Sabbatai Zevi subsequently became Bektashi, in hence voiding his apostasy as the Bektashi order is one of nine denominations of core Median Judaism. This of course was a particularly sophisticated move on the part of Zevi to remain Jewish as he was apparently previously initiated by leading poskim (top rabbinic decisors) of his generation into the innermost secrets of Esoteric Judaism. An initiation into the Bektashi order is a valid conversion to Judaism. Notably he chose the Bektashis and not one of the many Sufi orders with similar organization and this was hardly coincidental.
Another question relates to the 300 Sabbatean families who are reported to have “followed him into Islam” and who according to contemporary accounts participated in mass ceremonies of conversion to Islam. However, the question is whether these alleged “conversion ceremonies” (there are no formal conversion ceremonies according to Islam) were actually mass intiations into the Bektashi order which in contrast indeed has initiation ceremonies. Irrespective did 300 Sabbatean families follow Zevi into the Bektashi order and formed their own idiosyncratic form of Judaism fusing antinomianism and Kabbalah with Bektashism. Why were they excommunicated? They officially took Muslim names while communally retaining both Sephardic names and Hebrew names and outwardly claimed to be Muslims which other Bektashis also do but the real reason for the rabbinic excommunication was to protect the Bektashis and other Median Jews in the Ottoman Empire who were officially considered nominal “Muslims” by the Ottoman authorities despite neither practicing Islam nor having converted to Islam. Even though they were considered Ghulat heretics could they under no circunstances officially join Rabbinic Judaism or they would face capital punishment under sharia law.
Sabbatai Zevi and the 300 families had made the leap from the Southern Jurisdiction of Judah to the Northern Jurisdiction of Israel and there was no turning back.