As my Istanbul-bound Turkish Airlines flight from Toronto began its descent, I caught a glimpse of a mosque in the bright sunshine, one of nearly 3,000 mosques scattered around Istanbul.

A few days later, I visited one of Istanbul’s marquee tourist attractions, the Blue Mosque, which is adorned with several imposing domes and a profusion of minarets.

But of all the mosques I saw in this bustling and exotic city, the so-called Jewish mosque, found in an upscale neighbourhood of gleaming condominiums and fine shops, may well have been the most unusual one.

A plain two-storey building with a beige exterior, a single white minaret thrusting into the sky and such decorative touches as fancy stone flourishes and four stolid columns shielding the front door, this is a mosque frequented by the donmehs. Or so I was told by a reliable source in Istanbul.

Members of an intensely secretive community that shuns contact with outsiders, the donmehs are descendants of Jews who converted to Islam in the mid-17th century.

Their story beggars belief.

Shabtai Zvi, a rabbi and a kabbalist born in the Ottoman Empire town of Smyrna in 1626, reached adulthood when the approach of messianic times seemed tantalizingly imminent. In 1648, when he was 22, he told his followers he was the messiah. The rabbis of Smyrna, fearing his deviant creed and his charismatic powers, excommunicated him and his flock.

Banished from the Jewish community, he travelled the world, landing in Istanbul, previously known as Constantinople, in 1666. Arrested by the authorities on suspicion of conspiring to overthrow the sultan, he was given a stark choice: convert to Islam or face execution.

He converted, disillusioning many of his followers, who now considered him a false messiah. Ardent loyalists, consisting of some 300 families, embraced his new faith and referred to themselves as ma’aminim, or “true believers.”

Muslims called them donmehs, a Turkish word meaning “to turn.” Outwardly, like the Jewish conversos of Spain, the donmehs made a show of embracing their new religion, living by the Muslim calendar and praying in mosques. Privately, they hewed to certain Jewish rituals, incorporating elements of the Kabbalah and Sufism into their beliefs, and recited prayers in Hebrew and Ladino.

After Shabtai Zvi’s death in 1676, the donmehs splintered into three sects. The Yakubis scrupulously observed Sunni Muslim religious obligations. The Karakas, seemingly less devoted to Islam, attracted acolytes in Poland, including Jacob Frank, who eventually converted to Christianity. The Kapanci, a hybrid sub-group, adopted the practices of both the Yakubis and the Karakas.

By the 19th century, the donmehs were concentrated in Salonika, in present-day Greece, and were prominent in the textile, timber and tobacco trades. Clannish and insular, they lived in the same neighbourhoods, maintaining their own network of mosques, schools, communal courts and cemeteries.

Numbering upward of 15,000 when the 20th century dawned, the donmehs, having been branded incorrigible deviants by rabbis, were alienated from the Jewish community.

In 1914, the chief rabbi of the Ottoman Empire accused them of immorality, sexual promiscuity and perversity, dishonesty, blasphemy and trickery.

According to American historian Marc David Baer, the author of a book on the donmehs, they were increasingly assimilated into Muslim society as they internalized Islamic values and intermarried with Muslims. Indeed, some donmehs advanced into senior positions in the Ottoman bureaucracy and armed forces and in Sufi orders.

Drawn to progressive political ideas, they joined the ranks of the Committee for Union and Progress, or Young Turks, which pressed for reforms in the period leading up to World War I. Javid Bey, supposedly of donmeh origin, was the minister of finance in the first Young Turk government. 

It has also been alleged that Mustafa Kemal Ataturk, the founder of Turkey’s secular republic in 1923, was of donmeh descent as well. The allegation stems from the fact that Ataturk attended a school in Salonika established by Semsi Efendi, a donmeh who may have been one of his teachers.

With the end of Turkey’s war of national independence, a war that defined its current geographic boundaries, Turkey and Greece agreed to a massive population exchange. Upward of 500,000 Muslims in Greece were transferred to Turkey, while about one million Greek Christians in Turkey were sent to Greece.

As part of this historic exchange, the donmehs were resettled in Turkey, having been forced to leave Salonika after Salonika’s chief rabbi decreed that donmehs were Muslims rather than Jews. In the main, they settled in Istanbul, Izmir and Ankara, Turkey’s capital.

In retrospect, the donmehs were fortunate to have left Salonika. Two decades later, during the Nazi occupation, more than 90 per cent of the Jews in the city were deported to and murdered in concentration camps in Poland. The donmehs would have suffered the same fate had the Nazis classified them as Jews.

But Jews they definitely were not. During the Ataturk era, from 1923 to 1938, a donmeh descendant, Teyfik Rustu Aras, was Turkey’s ambassador to the Soviet Union and Britain and served as deputy foreign minister and foreign minister.

Oddly enough, however, the donmehs were affected by the punitive 1942  wealth tax imposed on minorities: Jews, Greeks and Armenians.

Today, nearly four centuries after Shabtai Zvi proclaimed himself the messiah, the donmehs have been completely absorbed into the Turkish Muslim fold. “There are few who keep the old traditions,” said Rifat Bali, an Istanbul historian who writes on Jewish topics. “The rest have been assimilated.”

Izak Kolman, an adviser to Turkey’s chief rabbi, claims there are three categories of donmehs in Turkey today. The first group denies its Jewish antecedents altogether. The second may still practise Jewish rituals in secrecy. The third openly acknowledges its Jewish ancestry.

The Jewish community has no formal connections with the donmehs. “We have no relationship with them,” said Kolman.

Nonetheless, hardline Islamists and conspiracy theorists claim that their descendants control and have controlled the Turkish government. They also insist that Turkey’s former foreign minister, Ismail Cem, was a donmeh, and that Prime Minister Recep Tayyip Erdogan is a secret Jew.

Judging by these assertions, the donmehs continue to cast a shadow on contemporary Turkey.

(Last in a series on Turkey).