Jehovah's Witnesses and the Church of Jesus Christ of Latter-day Saints, regarded by many Christians as cults, reported the largest membership increases in a year, according to the National Council of Churches' 2008 Yearbook of American & Canadian Churches.
Although Jehovah's Witnesses currently rank 25th in size with over 1.06 million members, they reported a 2.25 per cent increase in membership since the publication of the 2007 Yearbook. The Church of Jesus Christ of Latter-day Saints - also known as the Mormon church - grew 1.56 per cent and is listed by the NCC as the fourth largest 'church'.
Notably, however, both Jehovah's Witnesses and the Mormon church are not accepted within many Christian circles as part of the larger Body of Christ over a number of controversial beliefs that the two religions hold.
The identification of Jehovah's Witnesses as Christian is debated largely due to their rejection of the Trinity, which most Christians regard as a fundamental doctrine. Latter-day Saints, meanwhile, are often criticised for their belief in "divine" books of scripture, aside from the Bible, including the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price.
Mormonism was formally listed under "cults and sects" by the Southern Baptist Convention - the largest Protestant denomination in the nation - but was more recently categorised among "newly developed religions" on the North American Mission Board apologetics page.
Other bodies in the newly published top 25 largest churches list that reported membership increases include the Catholic Church with a 0.87 per cent increase; the Southern Baptist Convention with a 0.22 per cent increase; the African Methodist Episcopal Zion Church with a 0.21 per cent rise; and the Assemblies of God with a 0.19 per cent growth.
The greatest losses in membership were reported by the Episcopal Church, which dropped 4.15 per cent in members, and the Presbyterian Church (USA), which decreased by 2.36 per cent. Both denominations are currently wracked by theological differences and the issue of homosexuality.
American Baptist Churches in the USA and the Evangelical Lutheran Church in America also experienced large losses in membership, dropping 1.82 per cent and 1.58 per cent, respectively.
"Some will wish to argue that the slowing growth rate is evidence of an increasing secularisation of American postmodern society," said the Rev Dr Eileen W Lindner, editor of the Yearbook. "While such an explanation will satisfy some, caution in drawing such a conclusion is warranted."
Lindner also observed that churches are feeling the impact of the lifestyles of Millenials - people in their 20s and 30s - who attend church but resist becoming members.
The United Methodist Church saw a 0.99 per cent decrease but the mainline group remains the third largest church body with nearly 8 million members.
Only three of the top 10 largest churches are mainline Protestant churches; three of the top 25 are Pentecostal churches; and six of the top 15 are historic African American churches.
Largest 25 Churches (ranked by membership)
1. The Catholic Church - 67,515,016
2. Southern Baptist Convention - 16,306,246
3. The United Methodist Church - 7,995,456
4. The Church of Jesus Christ of Latter-day Saints - 5,779,316
5. The Church of God in Christ - 5,499,875
6. National Baptist Convention, U.S.A., Inc. - 5,000,000
7. Evangelical Lutheran Church in America - 4,774,203
8. National Baptist Convention of America, Inc. - 3,500,000
9. Presbyterian Church (U.S.A.) - 3,025,740
10. Assemblies of God - 2,836,174
11. African Methodist Episcopal Church - 2,500,000
12. National Missionary Baptist Convention of America - 2,500,000
13. Progressive National Baptist Convention, Inc. - 2,500,000
14. The Lutheran Church - Missouri Synod (LCMS) - 2,417,997
15. Episcopal Church - 2,154,572
16. Churches of Christ - 1,639,495
17. Greek Orthodox Archdiocese of America - 1,500,000
18. Pentecostal Assemblies of the World, Inc. - 1,500,000
19. The African Methodist Episcopal Zion Church - 1,443,405
20. American Baptist Churches in the U.S.A. - 1,371,278
21. United Church of Christ - 1,218,541
22. Baptist Bible Fellowship International - 1,200,000
23. Christian Churches and Churches of Christ - 1,071,616
24. The Orthodox Church in America - 1,064,000
25. Jehovah's Witnesses - 1,069,530