Religious fundamentalism is associated with Christian–Islamic conflicts globally, but the psychological reasons remain unexplored. Here, we show that fundamentalism is detrimental to interreligious relations because it makes Christians and Muslims alike reject common theological grounds and Abrahamic origins. Specifically, Study 1 demonstrated that such dual Abrahamic categories mediated the negative effects of fundamentalism on real monetary donations to outgroup children desperately in need (i.e., Save the Children Syria) among Christians but not Atheists. Of importance, this was the case only to the degree that Syrian children were perceived as Muslims and, hence, as part of an Abrahamic outgroup. Using a double-randomized experimental design, Study 2 demonstrated the causal effects of religious fundamentalism on Abrahamic categorization and of Abrahamic categorization on mutual resource distribution bias among Muslims and Christians. Together, these studies suggest that religious fundamentalism fuels interreligious conflicts because it crucially impacts basic categorization processes, with subsequent negative effects on intergroup relations.