Christianity pledges the peaceful acceptance of attacks but elicited numerous intergroup conflicts. This contradiction may arise from basic group-processes, including the rejection of intergroup criticism. Catholic Christians (Experiment 1, N = 322, online; Experiment 2, N = 252 in-person survey during Church activities) responded to criticism of their faith that was either attributed to a fellow Catholic or a Protestant commenter. Criticism from a Protestant (vs. Catholic) source was rated to be less constructive, more threatening, and ascribed worse commenter characteristics. Outgroup criticism did not change support for the unity of Christians (ecumenism, Experiment 1) but increased the likelihood of praying for the commenter (Experiment 2). The content of outgroup-directed prayers was hostile. Effects emerged independent of reported fundamentalism and in a context highly conducive to inter-denomination debate. These findings demonstrate that religious group processes can evoke counter-normative defenses, thus spurring conflict. Understanding these processes may help mend divided societies.