The paper traces the multiple factors through which a fluid and at times ambiguous religious identity among the Jews in Iran was transformed into a rigid one in the course of the 20th century. The Iranian province of Khorasan is a fruitful example for my argument, as at the beginning of 19th century, it was home to Sufi brotherhoods, where Jews and Muslims shared prayers, food, and literature. Although this was an asymmetrical relationship in the sense that Jews adapted to the majority Muslim culture, there were spaces of shared spirituality and culture, in which religious denominationalism played a minor role. In 1839, there was a riot in Mashhad, the capital city of Khorasan, in the course of which parts of the Jewish community were forced to convert to Islam. However, among those who converted, many kept their Jewish traditions secretly. They lived again more openly as Jews from the 1920s onwards. In the 1950s, the Jews of Mashhad left to Tehran, and later on to Israel and the United States. Since the 1980s, the community has increasingly reframed its religious identity: rather than to acknowledge the simultaneous practice of Jewish and Muslim identity and traditions, as well as the switching between different religious identities according to time and place, Mashhadi identity is structured according to a rigid dichotomy between a secret/true (Jewish) one and an outward/fake (Muslim) religiosity. In passing on their tradition, the Mashhadi community has resorted to a modern and Western understanding of being “Jewish”, a process that resulted in the reframing of their past according to this new framework of religious boundaries and identities. In reevaluating this history, I am not aiming to prove what was the “truth” or who was "really" Jewish. Rather, the history of the Jews of Mashhad is an important example for showing how ambiguous and dual religious identities were possible in a society, in which religion was one of the dominant factors in public life. Acknowledging this simultaneity will allow us to understand how public and private religion evolved in 19th century Iran, and to understand how the Jewish communities were part of these dynamics.