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 , 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. Private affluence can coexist with public squalor, and wealth alone is insufficient to guarantee widespread security.

Our theory is not deterministic or teleological. Even in affluent stable democracies, people can feel suddenly vulnerable from natural or manmade disasters, severe economic downturns, or personal tragedies. Within rich nations, certain sectors remain most at risk, typically the elderly, as well as poorer groups and ethnic minorities. Moreover, we agree with religious market theorists that contingent factors can also affect patterns of religiosity in particular contexts; the charismatic appeal of specific spiritual leaders can convert or mobilize their congregation, while conversely states can repress or persecute religious expression, as in China. In the long term and in global perspective, however, our theory predicts that the importance of religion in people's lives will gradually diminish with the process of human development. Moreover, it does so most dramatically during the first stage of human development, as nations emerge from low-income agrarian economies into moderate-income industrial societies with basic welfare safety nets safeguarding against the worst life-threatening risks; and, for reasons discussed in , this process does not reverse itself, but becomes less pronounced during the second stage, with the rise of postindustrial societies.