The study presents an overview on population dynamics of the two main religious affiliations in Nigeria: Christian and Muslim religions. The population of the two groups was estimated at 30-year intervals (1930, 1960, 1990, 2020). Data used for the reconstruction came from three population censuses and from ten demographic sample surveys. Population growth rates of Christians and Muslims were compared with estimates of net fertility derived from the same demographic surveys over the 1960 to 2020 period. Results were overall consistent and showed major trends over time: the rise of Christian religions and the fluctuations of Muslim religions as main affiliation, and as a consequence the apparent decline of African traditional religions. Between 1980 and 2010, both monotheist religions shared approximately half of the population. However, since 1995 the growth rate of the Muslim population became higher than that of the Christian population. In particular, population growth in the Northern part of the country, mainly Muslim, was outstanding. Recent trends could have serious implications in the future, and in particular could lead to demographic imbalance between the two groups, could raise serious environmental issues, especially in the North, and could have numerous political and social consequences.
