Source: Social scientists have a long history of predicting the demise of religion. Auguste Comte, Emile Durkheim, Sigmund Freud, and Karl Marx envisioned the decline of organized religion and the rise of the religiously unaffiliated. More recently the idea that the unaffiliated population will increase has been promoted using mathematical models of social group competition (Abrams, Yaple et al. 2011) and assumptions that growing economic development will lead to evolution away from religion (Barber 2012). But these predictions did not take demography into account − specifically, that patterns in global population growth favor those who have religious affiliation (Norris and Inglehart 2004; Kaufmann 2010). Our new demographic analysis finds that affiliated women have more children than unaffiliated women − nearly a full child more per woman, on average, worldwide. In addition, the global median age of affiliated women is six years younger than unaffiliated women, so they have more potential years of childbearing and living ahead. We project these demographic characteristics will result in a more religiously affiliated global population in coming decades. Although current patterns of religious switching favor the unaffiliated, they are insufficient at the global level to offset the demographic advantages of the affiliated. Demographic projections of affiliated and unaffiliated populations were not possible in the past because scholars had not collected the worldwide data necessary to make them. We assembled a database of religious composition and demographic characteristics, based on more than 2,500 censuses, surveys, and population registers for 198 countries and territories that make up 99.98% of the world’s population. Using this database, we made demographic projections of religiously affiliated and unaffiliated populations for the period from 2010 to 2050. Our global projections assume the continuation of recent switching patterns that strongly favor the unaffiliated, Demographic Research: Volume 32, Article 27 http://www.demographic-research.org 831 particularly in North America, Europe, and Latin America, where we project the unaffiliated will grow as a share of those regions’ populations. But if current trends continue, the religiously unaffiliated will decline as a percentage of the world’s overall population, from 16.4% in 2010 to 13.2% in 2050. Even when we model a 50% increase in current rates of switching, tilting even more in favor of religious disaffiliation, the unaffiliated share of the world’s population would still be expected to decline, falling to 14.3% in 2050.