There are estimated to be some 10,000 distinct and separate religions in the early twenty-first century. Information on the size and characteristics of their membership can be drawn from the various religious authorities themselves and from government data.
The Religion Megacensus
Major efforts are put into the collection of statistics by individual religious bodies. For example, Christian denominations periodically undertake a decentralized and largely uncoordinated global census of their members. The most extensive of these inquiries is conducted by the Roman Catholic Church, whose bishops are required annually to submit a detailed statistical report on their work. The entire data collecting operation, performed to some degree by all religious bodies, has been termed the religion megacensus.
Since the twelfth century, the world's governments also have collected information on religious populations and practice. A question related to religion is asked in the decennial population censuses of over 120 countries (not including the United States and a number of European countries). Before 1990, this number was slowly declining as developing countries began dropping the question because it was deemed too expensive, uninteresting, or sensitive. Subsequently, the trend appears to have reversed. Thus Britain–which produced the world's first national census of religious affiliation (the Compton Census, in 1676)–included a religion question in the census of 1851 but none thereafter, until it reintroduced the question in the 2000 census. The question was considered the best way to get reliable data on non-Christian minorities.
A Summary Global Table
These two approaches, the religion megacensus and government censuses with questions about religious affiliation, produce an enormous volume of data. Table 1 presents a compact global overview derived from these data, showing estimated number of adherents by major religion for 1900, 1970, and 2000.
Categories and Data Problems
The starting point in any analysis of religious affiliation is the United Nations 1948 Universal Declaration of Human Rights, Article 18: "Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance." Since its promulgation, this group of phrases has been incorporated into the state constitutions of a large number of countries across the world and applied in census-taking practice. If a person states that he or she is a Christian (or Muslim, Hindu, Buddhist, etc.), then no one has a right to say he or she is not. Public declaration or profession must be taken seriously. The result should be a clearcut assessment of religious profession.
Data on religious affiliation obtained from government censuses can be strikingly different from those collected by the religious bodies themselves. For example, in Egypt, where the great majority of the population is Muslim, in government censuses every 10 years for the last 100 years some 6 percent of the population are reported to be Christians. However, based on church censuses the number of Christians affiliated to churches in Egypt amounts to 15 percent of the population. The main reason for this discrepancy appears to be a misclassification of Christians as Muslims in the government census, perhaps through pressure on the Christian minority.
Changes in Affiliation
Changes in total numbers of persons by religious affiliation result from the combination of three factors: (1) births and deaths–that is, natural increase;(2) conversion and defection; and (3) population movement.
Natural increase. The primary mechanism of change in religious affiliation globally is births and deaths. Children are usually counted as being of the religion of their parents (this is the law in Norway, among other countries). The change over time in any given community is most simply expressed as the number of births into the community minus the number of deaths. Many religious communities around the world experience little else in the dynamics of their growth or decline.
Conversion and defection. Nonetheless, it frequently happens that individuals (or even whole villages or communities) change allegiance from one religion to another (or to no religion at all). In the twentieth century, this change has been most pronounced in two general areas: (1) Tribal religionists, more precisely termed ethnoreligionists, have converted in large numbers to Christianity, Islam, Hinduism, and Buddhism; and (2) Christians in the Western world have defected in large numbers to become nonreligious (agnostics) or atheists. Both of these trends, however, had slowed considerably by the dawn of the twenty-first century.
Population movement. At the country level, it is equally important to consider the movement of people across national borders. From the standpoint of religious affiliation, migration can have a profound impact. In the colonial era in the nineteenth century, small groups of Europeans settled in Africa, Asia, and the Americas. In the late-twentieth century, people from these regions migrated to the Western world. Thus, in the United States, religions such as Islam, Hinduism, and Buddhism are growing faster than either Christianity or the nonreligious and atheists. This growth is almost entirely due to the immigration of Asians. In the Central Asian countries of the former Soviet Union, Christianity has declined significantly every year since 1990, due to the emigration of Russians, Germans, and Ukrainians.
Tables similar to Table 1 but giving more detailed categories, breakdowns of change over time, and projections for several decades ahead have been prepared for every country. These data tables and detailed descriptions of methodology–explaining how the formidable technicalities were resolved–can be found in World Christian Trends,ad30–2200 (Barrett and Johnson, 2001) and World Christian Encyclopedia (Barrett, Kurian, and Johnson, 2001). The underlying demographic data incorporate the updates (every two years since 1950) of the United Nations population database for all countries from 1950 to 2050, and for some 100 variables each. A summary report on the religion megacensus has been published annually in Encyclopaedia Britannica's Book of the Year since 1987.
Annuario Pontificio. Annual. Citta del Vaticano: Tipografia Poliglotta Vaticana.
Barrett, David B., and Todd M. Johnson. 2001. World Christian Trends,ad30–2200: Interpreting the Annual Christian Megacensus. Pasadena, CA: William Carey Library.
Barrett, David B., George T. Kurian, and Todd M. Johnson. 2001. World Christian Encyclopedia: A Comparative Survey of Churches and Religions in the Modern World, 2nd edition, 2 Vols. New York: Oxford University Press.
Eliade, Mircea, et al., eds. 1986. The Encyclopedia of Religion. New York: Macmillan.
Statistical Yearbook of the Church. Annual. Citta del Vaticano: Secretaria Status.
Todd M. Johnson
David B. Barrett
"Religious Affiliation." Encyclopedia of Population. . Encyclopedia.com. (January 21, 2019). https://www.encyclopedia.com/social-sciences/encyclopedias-almanacs-transcripts-and-maps/religious-affiliation
"Religious Affiliation." Encyclopedia of Population. . Retrieved January 21, 2019 from Encyclopedia.com: https://www.encyclopedia.com/social-sciences/encyclopedias-almanacs-transcripts-and-maps/religious-affiliation