Sabbatians and Frankists
In 1648 Shabtai Tzvi declared himself to be the long-awaited Jewish Messiah whilst living in the Ottoman Empire. Vast numbers of Jews, known as Sabbateans, believed him; but when under pain of a death sentence in front of the Turkish sultan Mehmed IV he became an apostate from Judaism by becoming a Muslim, his movement crumbled. Nevertheless, for centuries, small groups of Jews believed in him, and the rabbis were always on guard against any manifestations of this schism, always suspicious of hidden "Shebselach" (Yiddish for "little Sabbatians", a play on the word for "young dumb sheep"). Indeed, when the movement of Hasidism began attracting many followers, the rabbis were once again suspicious that this was Sabbatianism in different garb. It would take many centuries to sort out these complex divisions and schisms and see where they were headed.
After his mysterious death somewhere in the area of Turkish Albania, groups of Jews continued to be clandestine followers of Shabtai Tzvi even though they had outwardly converted to Islam, these Jews being known as the Donmeh. Jewish converts to Islam were, at times, therefore regarded with great suspicion by their fellow Muslims.
A few decades after Shabtai's death, a man by the name of Jacob Frank claiming mystical powers preached that he was Shabtai Tzvi's successor. He attracted a following, preached against the Talmud, advocated a form of licentious worship, and was condemned by the rabbis at the time. When confronted by the Polish authorities, he converted to Catholicism in 1759 in the presence of King Augustus III of Poland, together with groups of his Jewish followers, known as "Frankists". To the alarm of his opponents, he was received by reigning European monarchs who were anxious to see their Jewish subjects abandon Judaism and apostacise. The Frankists eventually joined the Polish nobility and gentry.
Read more about this topic: Jewish Schisms