Objective Several evolutionary theories contend that religion is critical to stabilizing risky coordination in situations where kin-selection and reciprocity might be insufficient. These theories argue that religions provide a mechanism for finding trustworthy group members by requiring individuals to signal their commitment to group norms through ritual behaviors that impose fitness costs. However, in anonymous contexts, the frequency or intensity of ritual participation usually cannot be observed; therefore, people use religious badges as proxies for ritual participation to facilitate cooperation. Research on American undergraduates showed that people adoring religious badges are indeed trusted more. Surprisingly, this effect extended even across religious divides, which is in contrast with the assumption that religions facilitate only within-group cooperation, often at the expense of competing groups. We test this conjecture in Mauritius and further investigate how religious identities interact with other essential group-membership signals, such as ancestry. Methods Ninety-seven Mauritian Christians and Hindus first rated the trustworthiness of 10 faces on a computer, which varied according to religious and ethnic identity. We digitally manipulated these faces to adore a religious badge that indicated ritual participation and that was either congruent or non-congruent with the faces' ethnic identity. Upon rating the faces, participants played a modified version of the Trust Game, in which they distributed endowed money among these faces. Results We find that markers of ritual participation increase monetary investments only among ingroups and not across religious divides. Moreover, out-group religious markers on faces of in-group ancestry decreased reported trustworthiness. Conclusion Our findings suggest that local ecologies influence the relationships between religion and trust. Markers of ritual participation were associated with cooperative behavior among religious ingroups, but seeing people adoring out-group religious badges may have opposite effects. We conclude that ritual behaviors evolved to facilitate within-group cooperation, and to some extent regulate coalitions.