Religious faith meets some extremely important psychological needs that have existed since the emergence of humankind: it provides a sense of security, safety, meaning, and comfort in a threatening world in which the only real certainty is that someday we will die. However, ideologically inspired violence—both terrorism and war—is also on the rise. According to research by Monica Toft, religious wars are more common than secular ones, and they are more brutal. They are also more likely to recur. Similarly, terrorists who claim to kill in the name of God are more common than secular ones today. Their numbers are increasing, and they kill more innocent civilians than their secular counterparts. How can it be that the same force—faith—that inspired Mahatma Gandhi and Martin Luther King Jr. also inspires religious killers? This question has haunted us for years, leading us to work on interviews, large-scale surveys, and psychological experiments that explore the links among religion, radicalism, and violence. This article comments on religious violence and peace as well as the function of religion.