When is shared religion mobilized in ethnic civil war? Most civil wars are fought between distinct ethnic groups, but in the last half century mobilization of religion is increasingly common, mostly in conflicts between ethnic groups that share religion. While the literature explains the role of religion in conflict onset—especially between groups segmented by religion—the reason why religion becomes an organizing principle in an ongoing violent conflict between ethnic groups that share a religion is understudied. We argue that in response to competition for followers in civil war, ethnic minority leaders sharing religion with majorities are increasingly likely to mobilize the religious identity through an oversized challenger’s winning coalition as the demographic balance of the ethnic groups evens out. We test this conjecture using the new A-Religion data on the religion, religious overlap, and relative sizes of all ethnic groups in civil wars from 1975 to 2015.