The Pew Forum on Religion and Public Life has just come out with Part 2 of their massive US Religious Landscape Survey. Their description of the results:
The survey finds that most Americans have a non-dogmatic approach to their faith; for example, most do not believe their religion is the only way to salvation. The survey also finds that religion is closely linked to political ideology.
The nonaffiliated are around 16%, but only about 4% are actually nonbelievers. Almost every American believes in God
(92%); the overwhelming majority indicate that they are “absolutely certain” about this.