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: