A large existing literature points to a cooperative advantage within groups: if individuals share a common group identity, they tend to work together, based on a common sense of trust, extended to all group members even if they are strangers otherwise. This group-based trust appears to be naturally occurring whenever a shared group identity is commonly known among group members and salient to them. The argument is made that this group-based trust can serve as an effective substitute for more generalized feelings of trust (in “most people”) to support collective action on a similarly large scale. The concept of Islam as a group identity is developed, in contrast to traditional definitions of Islam as a personal faith, and an argument is made that regular participation in religious group activities should be used as anindicator for this Islamic identity. The empirical distinction between personal religiosity and religious identity is illustrated in data from Turkey and across the Muslim world. In addition, the validity of group activities as an indicator of Islamic identity and in-group trust is tested and confirmed.