When the Jewish false messiah Shabbetai Tsvi converted to Islam in 1666—under threat of execution by the Ottoman sultan—most of those who had believed his claims became disillusioned; others remained committed in secret. But his most devoted followers joined him in becoming Muslim, a move they justified through Shabtai’s kabbalistic teachings. Their descendants, known as the Dönme, practice their own idiosyncratic form of Judaism in secret and remain a separate community in modern Turkey, where they are the subject of outlandish, and anti-Semitic, conspiracy theories. Türkay Salim Nefes writes:
Since the early 20th century, conspiracy theories have accused the Dönme of secretly manipulating Turkish society and politics.
The conspiratorial rhetoric initially emerged after Theodor Herzl [first tried to arrange a meeting with the sultan in 1896]. The Ottoman ruler, Abdulhamid II, did not grant the request, and in 1908, he was toppled by a coup d’état. Some conspiratorial accounts claimed that the coup was Jewish revenge for his refusal to “sell” Palestine [to Herzl]. . . .
[After 1945, the Turkish government’s] censorship of political groups decreased. This enabled right-wing and Islamist groups to circulate conspiracy theories about the community. . . . Until the 1990s, the conspiratorial accounts were confined to marginal right-wing circles, Turkish nationalists, and Islamists. During the 1990s and 2000s, the conspiratorial accounts became prevalent once again after [publication of] the works of a self-proclaimed Dönme, Ilgaz Zorlu. He wrote articles about the history of the group and advocated that Dönmes should convert back to Judaism. . . . In this period, not only right-wingers and Islamists but also some left-wingers . . . published conspiratorial accounts about the community.
Read more on Dayan Center: https://dayan.org/content/conspiracy-theory-culture-turkey-and-crypto-jews-donmes