Religious ritual invariably offers a sacrificial display of commitment to supernatural agents that is materially costly and emotionally convincing. Such ritual conveys willingness to cooperate with a community of believers by signaling an open-ended promise to help others whenever there is true need. These expensive and sincere displays underscore belief that the gods are always vigilant and will never allow society to suffer those who cheat on their promises. Such a deep devotion to the in-group habitually generates intolerance toward out-groups. This, in turn, leads to constant rivalry and unending development of new and syncretic religious forms.

Introduction: an evolutionary riddle, p. 16 - In Gods We Trust: The Evolutionary Landscape of Religion (2002)