What are the origins of differences in fertility and economic transition across African societies? This paper addresses this question by causally estimating average and heterogeneous effects of colonial-era Christian missions on human capital accumulation, fertility decline, and household wealth in Nigeria. Our identification strategy exploits discontinuities in mission stations around the borders of the Emirates of Northern Nigeria, where missionary activities were restricted by the colonial administration. We find that areas with greater historical missionary activities have higher levels of schooling, lower levels of fertility, and higher household wealth today. The long-run effect of missions is not found in areas with early access to government schools, and is larger for population subgroups-women and Muslims-that have historically suffered disadvantages in access to education. Importantly, we show that the restriction of missions from the Emirates of Northern Nigeria has led to a reversal of fortunes, wherein areas that were more prosperous and institutionally developed in the past are relatively poorer today. The analysis provides novel insights into the differential pace of demographic transition between Northern and Southern Nigeria, and highlights patterns consistent with the prediction of Unified Growth Theory, whereby an improved technological environment leads to greater demand for education, triggering a fertility decline, resulting in higher incomes per capita.