Of the 535 members of the 107th Congress, 150 are Roman Catholic, including 91 Democrats and 59 Republicans, the Maryland-based organization reported in Voice of Reason, its quarterly newsletter.
Baptist members increased by three members to 72, with 37 Republicans, 34 Democrats and one independent. Methodists saw the largest jump in numbers, adding six for a total of 65, including 39 Republicans and 26 Democrats.
The top religious affiliations have remained the same for decades, with Catholics as the largest religious bloc.
"They have been since 1964," said Albert Menendez, associate editor of the newsletter. "They're going to stay in first place probably forever."
Presbyterians in Congress increased by two members to a total of 49, including 32 Republicans and 17 Democrats. Episcopalians, dropped by one to 41, with 30 Republicans and 11 Democrats. Jewish members saw an increase of three members for a total of 37 -- 33 Democrats, 3 Republicans and one independent.
The number who identified themselves as "nondenominational Protestant" decreased by five to a total of 29, with 19 Republicans and 10 Democrats.
Rounding out the top 10, were 20 Lutherans (11 Democrats and nine Republicans), 15 Mormons (12 Republicans and three Democrats) and eight members of the United Church of Christ (five Republicans and three Democrats).
Geographically, Catholic members are often from the Northeast, the Great Lakes region and California. Baptists, Methodists and Presbyterians members are mostly from the South.
Seven members claimed no religious affiliation. There are no Muslim, Buddhist or Hindu members.
Americans for Religious Liberty is a group that monitors trends in religion and politics and supports church-state separation.