In this article, I provide a philosophical analysis of the nature and role of perceived identity threats in the genesis and maintenance of fanaticism. First, I offer a preliminary definition of fanaticism as the social identity-defining devotion to a sacred value that demands universal recognition and is complemented by a hostile antagonism toward people who dissent from one’s group’s values. The fanatic’s hostility toward dissent thereby takes the threefold form of outgroup hostility, ingroup hostility, and self-hostility. Second, I provide a detailed analysis of the fears of fanaticism, arguing that each of the three aforementioned forms of hostile antagonism corresponds to one form of fear or anxiety: the fanatic’s fear of the outgroup, renegade members of the ingroup, and problematic aspects of themselves. In each of these three forms of fear, the fanatic experiences both their sacred values and their individual and social identity as being threatened. Finally, I turn to a fourth form of fear or anxiety connected to fanaticism, namely the fanatic’s anxiety of and flight from the existential condition of uncertainty itself, which, at least in some cases, ground the fanatic’s fearfulness.