Most readers interested in Jewish history know something about the conversos, the Spanish and Portuguese Jews forced to convert to Christianity in the 14th and 15th centuries. In recent decades, historians have come to see their story not just as a tragic or heroic one—an affair of Jews forced to give up their faith, or contriving to remain faithful in secret—but as an important episode in the evolution of the modern world. Yirmiyahu Yovel argued last year in The Other Within: The Marranos that these “New Christians” were the first large group in European history to be effectively post-religious—free to define the world and its meaning for themselves, instead of accepting the definitions of rabbinic Judaism or medieval Catholicism. That Spinoza and Montaigne, those skeptical modern minds, were both descendants of conversos, and that New Christians played a major role in the economy of the New World, is seen as evidence that these Jewish converts helped to invent the secular world we live in.Much less is known, however, about a later, smaller, but perhaps even more intriguing group of Jewish converts, who emerged in the Ottoman Empire in the late 17th century. They were followers of the arch-heretic Sabbatai Zevi, who proclaimed himself the Messiah and set about abolishing major Jewish laws and customs. Despite, or because of, the blasphemous nature of his innovations—for instance, he declared that Tisha B’Av, the greatest day of mourning in the Jewish calendar, would henceforth be a day of celebration—Zevi attracted a large following across the Jewish world. But in 1666, Zevi was arrested by the Ottoman authorities and given the choice of converting to Islam or being executed. When he chose to convert, he left thousands of disillusioned believers behind him. Glückel of Hameln, the author of a famous autobiography, compared the experience to being pregnant for nine months, and then, instead of giving birth, only breaking wind.But a small group of Sephardic Jews, many of them descended from conversos, did not think that Zevi’s apostasy invalidated his mission. On the contrary, they decided to follow him by converting to Islam themselves, while continuing to believe in their messiah and follow his commandments. This group, totaling about 300 families, became known in Turkish as Dönme, “converts,” though they referred to themselves in Hebrew as Ma’aminim, “believers.” By the 1680s, the Dönme had congregated in Salonika, the cosmopolitan and majority-Jewish city in Ottoman Greece. For the next 250 years, they would lead an independent communal life—marrying within the community, doing business together, maintaining their own shrines, and handing down their secret traditions.In The Dönme: Jewish Converts, Muslim Revolutionaries, and Secular Turks, Marc David Baer has produced the first scholarly study of this group. That it is a scholarly work, limited in its scope and sticking closely to written archives, is something that Baer insists on, and with good reason. For while the Spanish conversos are now seen as an interesting historical phenomenon, and it is even rather fashionable to claim converso ancestry, Turkey is still a part of the world where the anti-Semitic imagination runs wild.And because the Dönme played an outsize role at key moments in modern Turkish history, the myth of their secret Jewish power has itself become powerful. As Baer writes in his introduction, there have recently been bestselling books in Turkey claiming that everyone from the current prime minister, the religious Muslim Recep Tayyip Erdogan, to Mustafa Kemal Ataturk, the revered founder of modern secular Turkey, are secretly Jewish. “Ghost Jews haunt the Turkish popular imagination,” as Baer puts it.This makes it a delicate matter to write about the Dönme. In fact, Baer says, most of the descendants of Dönme whom he interviewed for the book asked him not to use their names. “Although many believe conspiracy theories about the Dönme,” Baer writes, “very few know the real character and history of the group.” His book, perhaps deliberately, will not raise the profile of the Dönme very much. Not only is it an academic book, published by Stanford University Press, but Baer says very little about the origin of the Dönme, or about their religious beliefs and practices—matters that many Jewish readers would be curious about.In fact, he emphasizes that the Dönme, unlike the conversos, do not really merit the title “crypto-Jews.” They were not Jews who pretended to be Muslims, but a sect of their own, whose beliefs and practices were actually further from Judaism than from Islam. Because they were originally followers of Sabbatai Zevi, mainstream Sephardic Jews wanted nothing to do with them. Baer quotes one rabbinical opinion from 1765, declaring that “there is no difference between them and the Gentiles at all, transgressing against all that is written in the Torah, certainly taken for Gentiles in every matter.” The Dönme followed the Muslim calendar and prayed in mosques, though they dissented privately in some ways. For instance, while they fasted during the daytime on Ramadan, like all Muslims, they deliberately broke the fast a few minutes early, thus signaling their independence.For the most part, however, Baer has little to say about Dönme origins and religious beliefs. He focuses instead on the Dönme in the late 19th and early 20th centuries, and on institutions like schools and businesses that are officially documented. When some exotic feature of Dönme practice does come into view—for instance, the allegation that they celebrated a certain holiday with orgies—Baer is quick to note that such sexual sins are always imputed to religious schismatics, in the Muslim world as in the Christian world. (The word “buggery,” for instance, derives from the medieval Christian heretics known as Cathars, who were from Bulgaria.)In Baer’s hands, the story of the Dönme becomes, instead, a rather familiar modern morality play—a story of strangeness annihilated by the pressure of sameness. For centuries, the Dönme lived their communal life in Salonika without interference from the Ottoman Empire, which accepted them as Muslims and did not inquire too closely into their private convictions. That began to change in the late 19th century, as the corrupt and cosmopolitan empire started to turn into a modern national state. The Dönme, who were prominent in the tobacco and textile industries, were initially strong supporters of political reform. Baer discusses the pro-reform articles in Dönme newspapers and literary magazines and notes that Dönme schools in Salonika were some of the most progressive in the Empire. (Ataturk attended one of those schools, though the evidence seems to prove that he was not a Dönme himself.)Most important, several Dönme were leading members of the Committee for Union and Progress, the revolutionary party known as the Young Turks, who in 1908 forced the Sultan to grant a constitution. The Dönme, like Jews and Freemasons, sympathized with the CUP’s scientific, reformist program, though Baer emphasizes that the CUP was not a Dönme party—any more than the Russian Bolsheviks, though they included many Jews, were a Jewish party. Even so, some prominent Young Turks were Dönme, including the editor of the Party’s newspaper and the finance minister in the new CUP government.This newfound prominence came just as the old Dönme community in Salonika was uprooted. In 1912, the city was conquered by Greece, which changed the name to Thessaloniki and set about expelling the Muslim population. The Dönme were forced to abandon their shrines and homes, and most of them resettled in Istanbul. Now in the public eye as never before, they were the subject of a number of muckraking newspaper articles and books, which Baer examines. In 1919, one anonymous publication accused them of being inbred to the point of biological degeneracy: “Muslims who give their daughters in marriage to those among whom tuberculosis and neuralgia/neural disorders are widespread are committing murder,” the writer warned. At the same time, the Dönme were said to be “always occupied with commerce. Because they do not consider others to be human, they consider it among the laws and praiseworthy qualities of their religion to cheat other nations with various intrigues and schemes.”It is impossible to miss how closely such anti-Dönme rhetoric resembles anti-Semitic rhetoric, both the modern biological type and the traditional economic type. The Dönme may not have been Jews, but they functioned in the Turkish imagination as Jews—they were clannish, untrustworthy outsiders, who were actually more threatening than the actual Jews because they had so long pretended to be Muslims. In the 1920s, then, as the modern Turkish state was founded on a racial and nationalist basis, the Dönme came in for severe discrimination. Even one prominent Dönme journalist wrote that “this problem must be decisively liquidated,” so that those Dönme “who are truly Turkish and Muslim [can be] distinguished in public opinion … and saved from the necessity of carrying on their back the social stain.”Soon enough the “problem” was liquidated, through intermarriage and assimilation. By mid-century, the Dönme had begun to disappear as a separate community, and today, Baer writes, the old Dönme cemetery in Istanbul is “the only place where the existence of the Dönme is really manifested as a distinct group.” Still, he notes, they were spared an even worse fate. As Muslims, the Dönme were expelled from Salonika in the 1910s, despite their protests. If they had been allowed to remain, they would have come under Nazi occupation during World War II, and given the Nazi racial definition of Jewishness, they would certainly have been sent to Auschwitz. In the terrible 20th century, The Dönme shows, there was no safe place for those on the margins.Adam Kirsch is a poet and literary critic, whose books include The People and the Books: 18 Classics of Jewish Literature.