How does a once familiar and benign ethnoreligious community become a stranger and a threat? This article examines the underlying causal mechanisms driving rival ethnoreligious factions within pluralistic polities to frame each other as threats to their relative security, power, and status. Drawing on complementary theories from critical security, religious, and nationalism studies, I develop a framework that captures and explains the processes and dynamics through which threatening conceptions and narratives about the ethnoreligious others are constructed, socialized, and legitimized over time. To theoretically probe and empirically demonstrate the utility of this framework, I examine how the collective imagined insecurities among Muslim and Christian communities in Indonesia have crystallized into tangible security threats using the interpretive process tracing method. Evidences produced from my theoretical and empirical analyses using the novel qualitative data I gathered from my field research reveal that this chauvinistic, zero-sum phenomenon proceeds via a three-phase othering causal mechanism comprised of cultivation of hostile emotive effects of ethnoreligious nationalism, securitization of othered ethnoreligious groups using hostile symbolic predispositions, and sacralization of hostile perceptions of indivisible ethnoreligious identities and homelands.