If there are atheists and agnostics serving in the halls of Congress, there are none who will openly admit to it. According to the latest Pew Research survey of U.S. Senators and Representatives, no member of Congress admitted to having no religious affiliation.
The most popular religious label in Congress is Protestant, followed by Catholic:
The 112th Congress, like the U.S. public, is majority Protestant and about a quarter Catholic. Baptists and Methodists are the largest Protestant denominations in the new Congress, just as they are in the country as a whole. ...
Perhaps the greatest disparity between the religious makeup of Congress and the people it represents, however, is in the percentage of the unaffiliated -- those who describe their religion as atheist, agnostic or "nothing in particular." According to information gathered by CQ Roll Call and the Pew Forum, no members of Congress say they are unaffiliated. By contrast, about one-sixth of U.S. adults (16%) are not affiliated with any particular faith. Only six members of the 112th Congress (about 1%) do not specify a religious affiliation, which is similar to the percentage of the public that says they don't know or refuses to specify their faith.
For more analysis of the survey, click here.