How are adherents counted?
There are five main methods for determining the number of adherents in a faith group:
Organizational reporting: Religious bodies (such as churches or denominations) are asked how many adherents or members they have. This is the simplest and least expensive method, but it can be highly unreliable. Different faith groups measure membership differently. Some count as members only those who are actively attending services or who have passed through a lengthy initiation process. Others groups count all who have been baptized as infants and are thus on the church records, even though some of those people may have joined other faith groups as adults. Some groups over-report membership and others under-report membership. When asked what religion they consider themselves to be a part of, many may name a religion that does not have them on their rolls. In the United States, for instance, three times as many people claim to be Unitarian Universalists than are actually on church records.
Census records: Many countries periodically conduct a comprehensive household-by-household census. Religious preference is often a question included in these census counts. This is a highly reliable method for determining the religious self-identification of a given population. But censuses are usually conducted infrequently. The latest census may be too old to indicate recent trends in religious membership. Also, many countries either have no accurate census data, or do not include questions regarding religious affiliation. It has been over fifty years since the United States included such a question in its national census, but Canada, India, New Zealand, Australia and other countries have very thorough, recent census data on the topic.
Polls and Surveys: Statistical sampling using surveys and polls are used to determine affiliation based on religious self-identification. The accuracy of these surveys depends largely on the quality of the study and especially the size of the sample population. Rarely are statistical surveys of religious affiliation done with large enough sample sizes to accurately count the adherents of small minority religious groups.
Estimates based on indirect data: Many adherent counts are only obtained by estimates based on indirect data rather than direct questioning or directly from membership roles. Wiccan groups have traditionally been secretive and often their numbers can only be estimated based on magazine circulations, attendance at conferences, etc. The counts of many ethnic-based faith groups such as tribal religions are generally based on the size of associated ethnic groups. Adherents of some tribal religions (such as Yoruba) are sometimes counted simply by counting the members of the tribe and assuming everybody in it is an adherent of the religion. Counts of Eastern Orthodox religious bodies are often done the same way. Such estimates may be highly unreliable.
Field work: To count some small groups, or to count the number of adherents a larger group has within a specific geographical area, researchers sometimes do "field work" to count adherents. This is often the only way to count members of small tribal groups or semi-secretive, publicity-shy sects. Field work may involve contacting leaders of individual congregations, temples, etc., conducting interviews with adherents, counting living within enclaves of the group, or counting those participating in key activities. There is substantial overlap between "estimates" and "field work."
For the purposes of this list of major religions, we have used adherent counts or estimates based on self-identification. We have also favored inclusive rather than exclusive adherent counts (meaning all people who are part of a religious community, children as well as adults, rather than "full communicants"). It should be remembered, however, that self-identification is not the only legitimate measure of a religious group's size. In collecting census or survey data based on self-identification statisticians find that nearly everybody claims to belong to a religion. Some people claiming membership in a certain denomination may actually attend religious services regularly, contribute resources to the group, and be influenced by its teachings. Other people may name the denomination, but choose it as their religion only because they recall its name as the church their grandfather attended as boy. Detailed analysis of the size of individual groups requires a knowledge of both self-identification data as well as data based on organizational reporting.
Finally, let me make it clear that these definitions are simply working definitions for the purposes of making this list. They should not be taken as definitive outside of this context. Many of our reasons for defining the parameters as we have done have to do with the availability of data. Other definitions and parameters may be more meaningful or useful in other situations.
Notes on the Size of Specific Religions
NOTE: The following material is not intended to provide descriptions or summaries of these religions. This material is only intended to describe the reasoning for listing groups as "major religions" and determining their general size. (To learn more about these faith groups, we suggest the Adherents.com links page, which will direct you to other web sites.)
Christianity: David B. Barrett's World Christian Encyclopedia (1994 update) gives an oft-cited figure of 1.9 billion Christians (or about 33% of the world population), and projected that by the year 2000 there will be 2.1 billion Christians in the world. The 2001 edition of the World Christian Encyclopedia stated there were 2.1 billion Christians in the world, or 33% of the total population. Regardless of the degree of accuracy of this figure, Christianity, if taken as a whole, is unarguably the largest world religion - the largest religion in the world. (Keep in mind that although Christianity is the world's largest religion, it is an umbrella term that comprises many different branches and denominations.)
See also: The Christian Family Tree by Rev. Epke VanderBerg (Episcopal minister, Grand Rapids, MI); Classifying Protestant Denominations (General Social Survey project directed by James A. Davis and Tom W. Smith. Funded by the National Science Foundation.); Largest Christian Populations (lists the Top 10 Countries with the Most Christians and the Top 10 U.S. Most Christian U.S. States); Famous Christians.
For statistical purposes: Groups which self-identify as part of Christianity include (but are not limited to): African Independent Churches (AICs), the Aglipayan Church, Amish, Anglicans, Armenian Apostolic, Assemblies of God; Baptists, Calvary Chapel, Catholics, Christadelphians, Christian Science, the Community of Christ, the Church of Jesus Christ of Latter-day Saints ("Mormons"), Coptic Christians, Eastern Orthodox churches, Ethiopian Orthodox, Evangelicals, Iglesia ni Cristo, Jehovah's Witnesses, the Local Church, Lutherans, Methodists, Monophysites, Nestorians, the New Apostolic Church, Pentecostals, Plymouth Brethren, Presbyterians, the Salvation Army, Seventh-Day Adventists, Shakers, Stone-Campbell churches (Disciples of Christ; Churches of Christ; the "Christian Church and Churches of Christ"; the International Church of Christ); Uniate churches, United Church of Christ/Congregationalists, the Unity Church, Universal Church of the Kingdom of God, Vineyard churches and others. These groups exhibit varying degrees of similarity, cooporation, communion, etc. with other groups. None are known to consider all other Chrisian sub-groups to be equally valid. David Barrett, an Evangelical Christian who is the compiler of religion statistics for the Encyclopedia Britannica and the World Christian Encyclopedia, includes all of the groups listed above in the worldwide statistics for Christianity.
Contemporary sociolgists and religious leaders generally consider pan-denominational classifications based not on historical denominational divisions but on current theological positions, organizational alignments, etc. to be more relevant. Such groupings include: Evangelicals, Pentecostals, "Great Commission Christians", "C. S. Lewis Christians", Liberal Protestants, Conservative Protestants, Fundamentalists, etc.
Islam: Contemporary figures for Islam are usually between 1 billion and 1.8 billion, with 1 billion being a figure frequently given in many comparative religion texts, probably because it's such a nice, round number. That figure appears to be dated, however. Relatively high birth rates in Muslim countries continue to make Islam a fast-growing religion. The largest and best known branches of Islam are Sunni and Shi'ite. More.
Many Muslims (and some non-Muslim) observers claim that there are more practicing Muslims than practicing Christians in the world. Adherents.com has no reason to dispute this. It seems likely, but we would point out that there are different opinions on the matter, and a Muslim may define "practicing" differently than a Christian. In any case, the primary criterion for the rankings on this page is self-identification, which has nothing to do with practice.
Smaller groups within Islam include Sufis (although some Sufis regard their practice of Sufism as pan-denominational or non-denominational), Druze, the U.S.-based Nation of Islam (previously known as "Black Muslims"), and Ahmadiyya. As is true with all major religions, there are adherents within all branches of Islam who consider some of or all of the other branches heterodox or not actually part of their religion. But these classifications are based primarily on historical lineage and self-identification. Protestations and disagreements based on exclusivistic internal concepts of belief or practice are normal, but are largely immaterial with regards to historical, taxonomic and statistical classification.
Secular/Nonreligious/Agnostic/Atheist: This i