Writers on the future of religion are usually drawn to extreme portrayals of decline or revival of religion. However, the world's religious situation is replete with detailed information, drawn from enormous data collections on religious affiliation and questions about religion in government censuses. Quantitative tools, utilizing this information in the context of demo graphy-births, deaths, conversions, defections, immigration, and emigration, provide a more nuanced view of mankind's religious future. Alternate futures of religious affiliation can be produced by extrapolating explicit assumptions related to these six areas. Demographic trends coupled with conservative estimates of conversions and defections envision over 80% of the world's population will continue to be affiliated to religions 200 years into the future. (C) 2004 Published by Elsevier Ltd.