Transnational Islamic communities authorize different forms of Islam while negotiating between the Islamic tradition and the liberal and secular norms of the European public sphere. This article compares how the production of religious authority within two Turkish Islamic communities—Gülen and Süleymanl? —shape two distinct types of Islam—civil and traditionalist. Suleymanli is a branch of Naqshibandiyya Sufi order and Gülen community has emerged as a revivalist faith-based movement. Islamic community (cemaat in Turkish) refers to an informal network based on shared interpersonal experience in socio-religious activism and discourse, which claims to represent “true Islam” through its activities and institutions. Each Islamic community is constituted in periodic face-to-face gatherings such as reading circles and rituals in which religious knowledge is produced and disseminated. The interaction of a religious corpus of assertions, their media of representation, and forms of social organization during these gatherings authorizes multiple forms of Islam based on different “criteria of Islamic validity and priority”. Religious authority involves the individuals, positions, actions, and sources that represent collective identity and preserves group cohesion by controlling and disciplining individuals through their “criteria of validity and priority” who become members or dropouts—that is through boundary making. The field of activism each organization concentrates such politics, education, or religious instruction defines group distinction and promotes the type of knowledge that religious authority embodies and distributes through boundary making. The comparison of the approaches of these two communities to Islamic activism and practice in Europe suggests that Islam is neither a “seamless essence” nor endlessly malleable, but a negotiation between Islamic tradition and the particular circumstances of Muslims. The emergence of “European” adaptations of the Islamic tradition requires an analysis of how Islam is authorized in Europe rather than simply what “European Islam” is or who speaks on behalf of it, individually or communally.