Unless otherwise designated, the demographic figures in this book, both in the colour section and in the tables throughout, are from the World Christian Database (Leiden/Boston: Brill). This essay offers a concise explanation of methods and sources related to the database. It is adapted from longer treatments in Todd M. Johnson and Brian J. Grim, The World's Religions in Figures: An Introduction to International Religious Demography (Oxford: Wiley-Blackwell, 2013) and Todd M. Johnson and Gina A. Zurlo, World Christian Encyclopedia, 3rd edition (Edinburgh: Edinburgh University Press, 2019). The World Christian Database (WCD) includes detailed information on 45,000 Christian denominations and on religions in every country of the world. Extensive data are available on 234 countries and 13,000 ethno-linguistic peoples, as well as on 5,000 cities and 3,000 provinces. Information is readily available on religious activities, growth rates, religious literature, worker activity and demographics. Sources are evaluated and reviewed on a weekly basis by a professional staff dedicated to expanding and updating the WCD, and the database is updated quarterly.

The Right to Profess One's Choice

The starting point of this methodology is the United Nations 1948 Universal Declaration of Human Rights, Article 18:

Since its promulgation, this group of phrases has been incorporated into the state constitutions of a large number of countries across the world. This fundamental right also includes the right to claim the religion of one's choice, and the right to be called a follower of that religion and to be enumerated as such. The section on religious freedom in the constitutions of very many nations uses the exact words of the Universal Declaration, and many countries instruct their census personnel to observe this principle. Public declaration must therefore be taken seriously when endeavouring to survey the extent of religious and non-religious affiliation around the world.

Religious Demography

The origins of the field of religious demography lie in the church censuses conducted in most European societies. For many years and in many countries, churches produced the most complete censuses of the population.