2000 data were collected by the Association of Statisticians of American Religious Bodies (ASARB) and include statistics for 149 religious groups, including number of churches and adherents. Dale E. Jones, Sherri Doty, Clifford Grammich, James E. Horsch, Richard Houseal, Mac Lynn, John P. Marcum, Kenneth M. Sanchagrin and Richard H. Taylor supervised the collection. These data originally appeared in Religious Congregations & Membership in the United States, 2000, published by the Glenmary Research Center.

The Association of Statisticians of American Religious Bodies (ASARB) invited all religious bodies that could be identified as having congregations in the United States to participate. Final totals include information from 149 Christian and other religious bodies. Participants included 139 Christian denominations, associations, or communions (including Latter-day Saints and Unitarian/Universalist groups); two specially defined groups of independent Christian churches; Jewish and Islamic estimates; and counts of temples for six Eastern religions. The 149 groups reported 268,254 congregations with 141,371,963 adherents, which is 50.2% of the population of 281,421,839. There are 14 non-participating religious bodies that reported more than 100,000 members to the Yearbook of American and Canadian Churches, 2000, including all historically African American denominations. These groups reported a combined membership of 31,040,360 in the Yearbook, which is not reflected in the congregations and membership data. The lack of historically African American denominations should be noted when referencing the number of total adherents or denominations in an area.

The RCMS collection reports a measure of members and adherents. Members include only those who are designated as full members by the congregation. Congregational "adherents" include all full members, their children, and others who regularly attend services or participate in the congregation. When religious groups reported only adult membership, the following formula was used to derive the number of adherents: The total county population was divided by the total county population less children 13 years and under (derived from census), and the resulting figure was multiplied by the confirmed members. Using adherents allows for more meaningful comparisons between groups that count children as members (e.g., Catholics) and those that don't (e.g. Baptists).

For total adherents we report both unadjusted totals and adjusted totals. The "unadjusted totals" come from the 2000 adherent totals reported in the RCMS collection. While quite comprehensive, this data excludes most of the historically African-American denominations and some other major groups. As a result, these numbers will underestimate the total adherence rate, particularly in areas with a large African-American population. The "Adjusted Totals" include all adherents in the denominations counted by the Association of Statisticians of American Religious Bodies (ASARB) and estimates adherent totals for the historically African-American denominations and other religious groups not listed in the ASARB totals. An article by Roger Finke and Christopher P. Scheitle (2005) reviews how these estimates were computed.

The Jewish totals and Muslim totals also rely on estimates. Jim Schwartz and Jeff Scheckner provided a list of Jewish populations by region, which is from the American Jewish Year Book 2001 (David Singer and Lawrence Grossman, 2001. New York: American Jewish Committee, pp. 253-280). Schwartz and Scheckner also supplied a count of synagogues by state and county. The list of synagogues was compiled by contacting Jewish denominations and other organizations, consulting all relevant websites, and obtaining lists from Jewish federations in large Jewish communities.

A telephone survey conducted by Ihsan Bagby identified the membership and number of adherents in over one third of Americas known mosques. At its simplest level, the non-reporting mosques were given the average number of adherents in the reporting mosques. Unlike most Christian churches, however, more than one mosque may claim the same person as an adherent. The following adjustment was made to account for a possible overlap of membership. When a mosque reported an adherence rate over eight times its average Friday attendance and there were other accessible mosques in the area, the number of adherents was lowered to eight times the number of attendees. Fifteen percent of the mosques reported an adherence/attendance ratio exceeding 8.0.

Christian groups in America are often placed into four main traditions: Evangelical Protestant, Mainline Protestant, Historically Black Protestant and Roman Catholic. There is wide variation within each of these four traditions. The most difficult to classify are evangelical and mainline Protestants, with evangelical groups generally being more theologically and socially conservative than mainline groups. Several sources were used for categorizing the many Christian groups into religious traditions, beginning with an article by Brian Steensland and colleagues entitled "The Measure of American Religion: Toward Improving the State of the Art" published in Social Forces (2000, 79: 291-318). This article places most denominations into one of above traditions. For Christian denominations not included in Steensland and colleagues' scheme, two additional sources were consulted: J. Gordon Melton's Encyclopedia of American Religions (8th edition) and Frank S. Mead and Samuel S. Hill's Handbook of Denominations in the United States (10th edition). In addition to those in Steensland and colleagues' classification scheme, separate categories were created for Muslims, Orthodox Christians, Jews, and other groups. The lack of data on historically African American denominations made inclusion of a Black Protestant category unnecessary. Using these sources, staff at the Association of Religion Data Archives (theARDA.com) placed each group into a Religious Tradition.

The 149 religious groups and their category placement are as follows: