In the globalization discourse, Christianity and Islam are often construed as representing two traditions that are conflicted and incompatible. This study engages the “clash of civilizations” discourse by examining Muslim-Christian differentials in the use of modern contraception in Nigeria, where Christians have a much higher contraceptive prevalence, and Tanzania, where Muslims are somewhat more likely to use contraception. Employing data from six nationally representative surveys conducted in the two countries between 1990 and 2004 and multilevel logistic regression, we find that the effects of religion remain strong but operate largely through the community religious milieu. Contraceptive use tends to be highest in religiously mixed areas, but the “optimal” religious makeup differs between the two nations, reflecting the historically shaped configurations of their religious expressions and politics.