The Gallup Organization came to that determination following their poll covering 2011.  52% of Americans identified themselves as Protestant/Other Christian and 23%
identified as Catholic.  Other affiliations included Mormon (1.9%), Jewish (1.6), and Muslim (0.5%).  One of the fastest growing group is those who identify themselves as non-affiliated/atheist/agnostic (15%).

According to the study, 92% of the 300,000 respondents say they believe in God, which suggests that “the lack of religious identity is not in and of itself a sign of the total absence of  religiosity.”  Also, separate surveys found an average of 55% of Americans saying that religion is very important to them.
–“Christianity Remains Dominant Religion in the United States,” by Frank Newport, December 23, 2011, appears on the Gallup Organization website (www.gallup.com).