During my stay at KHK I will work on the emergence and function of religious ideas as symbolic markers. I am relying especially on concepts of gene-culture co-evolution, anthropological insights about cooperation in large (pre-state) social groups, and social psychological research on the use of unifying symbols in groups. When people cooperate in large groups, they cannot identify group members by keeping track of their faces or names individually. A group that has a more efficient strategy of maintaining strong cooperation and loyalty among its members will prosper better than groups with less efficient strategies. From anthropological fieldwork we know how groups use symbolic markers, ranging from body painting, exchange of gifts, and marriage systems. Social psychological experiments have shown that virtually any mark can be used to create group identity. However, symbolic markers differ in their capacity of creating loyalty, altruism, and self-sacrifice. Some religious ideas seem to be particularly efficient symbolic markers. What makes some markers more efficient than others? What kind of religious ideas are suitable as symbolic markers and how do they evolve? A heuristic set of relevant factors includes memorability, easy differentiation from other markers (discriminability), symbolic potential that invites rich interpretation, as well as a potential for the attribution of personal significance to the symbol and the development of psychological attachment to it. I will investigate each of these factors in depth and pay special attention to examples from early Christianity and the religions of late antiquity, but will consider other traditions and historical periods, as well.