Secularization theorists tend to assume that secularization is linear, inevitable, caused by modernization, and found primarily in Western societies. In contrast, we develop a cyclical model of religious transformation which incorporates the process of secularization as well as other aspects of religious change, such as reform and revitalization, the creation of new religious movements, and the decline of traditional religions both historically and in contemporary non-Western societies. Data from Barrett's World Christian Encyclopedia (1982) are used to study religious changes between 1900 and 1980 in 200 nations and colonies, with 192 nations fitting into one of the stages of our model and 8 nations exhibiting reform tendencies. Decline of the majority religion is most characteristic of Tribal and Christian (especially Protestant) religions. Muslim, Buddhist and Hindu faiths are most resistant to decline. Atheism and non-religion have grown in most areas of the world, especially in Marxist and Western nations. Secularization theory does not provide a convincing explanation for the growth of irreligion. Religious changes are closely tied to political conditions in a nation. How do religions change, and why do such changes take place? What social conditions are conducive to religious change, and what conditions facilitate maintenance of the status quo? Are some religions more susceptible to growth or decline than others, and what accounts for these differences between religions? Is a decline in the established religion accompanied by growth in alternative religions? Are people becoming irreligious, and is religion losing its influence in the social world? These are some of the most crucial questions in the sociology of religion. In this paper we shall attempt some tentative answers to these questions and support our answers with a comparative study of religious change in 200 nations. One way to explain religious change is to treat it as a normal component of the culture and social structure of a society and to invoke established theories of social change to account for it. Many explanations of social change have been offered, including conflict, economic, cyclical, evolutionary, ecological, social-psychological, and equilibrium theories (Appelbaum, 1970). Processes such as invention, diffusion, differentiation, social strain, social movements, and innovation may apply to religion as well as other aspects of social life. Specifically religious processes, such as profanation, politicization, reform or renewal, repression of competing religions, and obsolescence of a particular religion, also need to be encompassed in the ideal theory of religious change.