Popular beliefs conflating ethnic identities with particular faiths can lead to marginalization of religiously and ethnically “other” and fuel ethnoreligious conflict even in secularized societies. Despite their importance to ethnoreligious conflict and coexistence, ideological conflations of faith and ethnos have previously received little scholarly attention. We fill the gap by introducing the concept of ethnodoxy: a collectively held belief system that rigidly links a group's ethnic identity to its dominant faith. We theorize this phenomenon at three levels: macro (the interplay between religion and ethnonational identities), micro (construction of ethnoreligious identities and imagined communities viewed from a social identity theory perspective), and meso (the nature and functions of popular ideologies and religiosities). Subsequently, ethnodoxy is conceptualized as an ideal-typical syndrome of six component beliefs. We further operationalize and verify the concept using representative national survey data from Russia. Findings show ethnodoxy's extent, coherence, dimensionality, and associations with religious and ethnic intolerance.