From Salonica, City of Ghosts: Christians, Muslims and Jews, 1430-1950, by Mark Mazower (Vintage, 2006), pp. 72-74:
Most of [Sabbatai] Zevi’s followers—like his right-hand man, the Gaza rabbi Nathan—never did convert [to Islam] and subterranean Sabbataian influences could be found among Jews as far afield as Poland, Italy and Egypt. In Salonica they lingered on for decades and only disappeared after the Napoleonic wars.
HUNDREDS MORE, HOWEVER, did actually follow Zevi into Islam—some at the time, and others a few years later—and by doing so they gave rise to what was perhaps one of the most unusual religious communities in the Levant. To the Turks they were called Dönmehs (turncoats [cf. Turkish döner kebap, Greek gyros for rotisserie meat]), a derogatory term which conveyed the suspicion with which others always regarded them. But they called themselves simply Ma’min—the Faithful—a term commonly used by all Muslims. (In Hebrew, the term is Maminim; in Turkish Mümin. Ma’min was a Salonica derivation.) There were small groups of them elsewhere, but Zevi’s last wife, Ayse, and her father, a respected rabbi called Joseph Filosof, were from Salonica, and after Zevi’s death, they returned there and helped to establish the new sect which he had created. By 1900, the city’s ten-thousand-strong community of Judeo-Spanish-speaking Muslims was one of the most extraordinary and (for its size) influential elements in the confessional mosaic of the late Ottoman empire.
Schism was built into their history from the start. Not unlike the Sunni-Shia split in mainstream Islam, the internal divisions of the Ma’min stemmed from disagreement over the line of succession which followed their Prophet’s death. In 1683 his widow Ayse hailed her brother Jacob—Zevi’s brother-in-law—as the Querido (Beloved) who had received Zevi’s spirit, and there was a second wave of conversions. Many of those who had converted at the same time as Zevi regarded this as impious nonsense: they were known as Izmirlis, after Zevi’s birthplace. Jacob Querido himself helped Islamicize his followers and left Salonica to make the haj in the early 1690s but died during his return from Mecca. As the historian Nikos Stavroulakis points out, both the Izmirlis and the Yakublar (the followers of Jacob Querido) saw themselves as the faithful awaiting the return of the Messiah who had “withdrawn” himself from the world; it was a stance which crossed the Judeo-Muslim divide and turned Sabbatai Zevi himself into something like a hidden Imam of the kind found in some Shia theology. A few years later, a third group, drawn mostly from among the poor and artisanal classes, broke off from the Izmirlis to follow another charismatic leader, the youthful Barouch Russo (known to his followers as Osman Baba), who claimed to be not merely the vessel for Zevi’s spirit but his very reincarnation.
Although they differed on doctrinal matters, the three factions had features in common. Following the advice of Zevi himself, whose eighteen commandments forbade any form of proselytism, they preserved an extreme discretion as a precaution against the suspicions and accusations which they encountered from both Turks and Jews. Even their prayers were suffused with mystical allusions to protect their inner meanings from being deciphered by outsiders.
Over time they developed a kind of mystical Islam with a Judaic component not found in mainstream Muslim life. While they attended mosque and sometimes made the haj, they initially preserved Judeo-Spanish for use within the home, something which lasted longest among Russo’s followers. They celebrated Ramadan, and ate the traditional sweets on the 10th of Moharrem, to mark the deaths of Hasan and Huseyn. Like their cooking, the eighteen commandments which they attributed to Zevi showed clearly the influence of both Muslim and Talmudic practice. (Was it coincidence that eighteen was also a number of special significance to the Mevlevi order?) They prayed to their Messiah, “our King, our Redeemer,” in “the name of God, the God of Israel,” but followed many of the patterns of Muslim prayer. They increasingly followed Muslim custom in circumcizing their males just before puberty, and read the Qur’an, but referred to their festivals using the Jewish calendar. Some hired rabbis to teach the Torah to their children. Although the common suspicion throughout the city—certainly well into the nineteenth century—was that they were really Jews (if of a highly unreliable kind), in fact they were evolving over time into a distinctive heterodox Muslim sect, much influenced by the Sufi orders.