This article presents the basic elements of a theory of religious secular competition. The theory claims that individuals in many societies may choose between religious and secular options, creating a situation of competition between religious and secular organizations. The paper assumes (bounded) rationality for individuals and organizations and describes religious and secular organizations, religious and secular goods, as well as three important parameters that influence religious secular competition: regulation of supply, regulation of demand and level of societal modernization. The theory of religious secular competition explains a whole range of different phenomena, such as variations in church going in US states, variation in the attractivity of monasteries, the late secularization of Ireland, the strong religiosity of agrarian societies or the success of megachurches since the 1960s.