We use the trust and the dictator games to explore the effects of religious identity on trust, trustworthiness, prosociality, and conditional reciprocity within a beliefs-based model. We provide a novel, rigorous, theoretical model to derive the relevant predictions, which are then tested in lab-in-the-field experiments in the Indian states of Bihar and Uttar Pradesh. We find strong evidence of the effects of religious identity on the belief hierarchies, and the chosen actions, of Hindu and Muslim subjects. Priming for a religious identity has little effect on Hindu subjects but it enhances religious polarization in beliefs and actions among Muslim subjects. There is taste-based discrimination but no statistical discrimination. All our underlying assumptions on beliefs, and their dependence on priming and identity are confirmed by the data, identifying a precise beliefs-based mechanism for the effects of religious identity. More religious subjects expect greater prosociality/reciprocity and often are more prosocial/reciprocal.</p