Q. How many people would say that they believed in God if they were able to answer with complete anonymity?
A. Several Pew Research Center surveys have included questions about belief in God, asking respondents “Do you believe in God or a universal spirit?” In our Religious Landscape Survey, a 2007 telephone survey with more than 35,000 respondents, 92% answered this question affirmatively.
However, in 2008, the Associated Press and Yahoo! News sponsored a series of online surveys conducted by Knowledge Networks among a nationally representative panel of Americans. The June 2008 wave of their poll included the same question about belief in God, and came up with very similar results (93% said yes, they believe in God or a universal spirit).
http://pewresearch.org/pubs/1770/ask-the-expert-pew-research-center