the theory developed in this book argues that the erosion of religious values, beliefs, and practices is shaped by long-term changes in existential security, a process linked with human development and socioeconomic equality, and with each society's cultural legacy and religious traditions. To clarify the core propositions, outlined earlier in Figure 1.1, we hypothesize that the process of societal modernization involves two key stages: (1) the transition from agrarian to industrial society, and (2) the development from industrial to postindustrial society. We argue that economic, cultural, and political changes go together in coherent ways, so that growing levels of existential security bring broadly similar trajectories. Nevertheless, situation-specific factors make it impossible to specify exactly what will happen in any given society: certain developments become increasingly likely to occur, but the changes are probabilistic, not deterministic. The modernization process reduces the threats to survival that are common in developing societies, especially among the poorest strata; and this enhanced sense of security lessens the need for the reassurance religion provides. The most crucial precondition for security, we believe, is human development even more than purely economic development: it involves how far all sectors of society have equal access to schooling and literacy, basic healthcare, adequate nutrition, a clean water supply, and a minimal safety net for the needy. Some developing countries have substantial national incomes derived from mineral and oil reserves, but many inhabitants remain illiterate, malnourished, or impoverished, due to social inequality, greedy elites, and governmental corruption.