Historian spend a lot of time developing trees on which to hang their ideas. This is fine to some extent, because good historians also find obscure subjects for their dissertations that explore the muddy boundaries.
This poorly surveyed terrain is what interests me and I hope would interest you, too. In our world, where DAESH doesn't just reject Shia Islam but makes it punishable by death, it's hard to imagine there was a time when Islam didn't just tolerate but allowed its heterogeneous sects to flourish.
For its many faults and brutality, the Ottoman Empire was very tolerant of minority sects, the followers of Tarikats and Sufi orders that did not conform at least in part to the Sunni tradition. This then is strange story of the Dönme, how they began and ended. They were followers of Shabbatai Tzevi, a man that claimed to be the Jewish Messiah. When confronted with the choice of death or conversion, he chose to convert, and was followed into Islam by some 250 Jewish families.
For over two hundred years, until the end of the 19th Century, these followers of Tzevi were considered Islamic without question, though they practiced endogamous marriages to their cousins, followed the 18 commandments of Tzevi, and seem to have retained at least some Jewish traditions, often by mixing Kabbalistic and Sufi beliefs.
That such a syncretic form of Islam hasn't been able to survive the demands of the ethnic, religious nation state is no surprise; the striving for purity of ethnic identity and religious beliefs fosters a conformity that such a complex identity cannot survive. Much like the daily destructon of bio-diversity, human tolerance seems to shrink every day. This is yet another cautionary tale about the pursuit of that intolerant genetic and social goal we call purity.
Baer's book is quite readable and incredibly well researched. I've stopped giving out many five star reviews because, in my opinion, having a well research tale isn't enough. The writing must also be exceptional, and Baer's book offers the reader very good but not exceptional prose. Nevertheless, as a one-time resident of Istanbul who has know people who had at least one parent from a Dönme background, I was struck but the great research that Baer has done. If you want to read about how this sect started, go to Gershom Scholem's scholarly book On Sabbatai Şevi. If you want an easier read about roughly the same period of the Dönme, read John Freely's book. But if you want a grand overview of the Döndme in the late 19th and early 20th century, when any number of Dönme were extremely influential members of the Committee for Union and Process, this is where you must look. If you want to find out how, after being accepted as Islamic for two centuries, they were suddenly looked at once again as being Jewish, to their detriment, this is the place to look. And if you want discover why this syncretic form of Islam/Judaism has been scattered to the winds, start here.