I think part of the answer is "belief in belief". - Edward Zwart from Bookmarklet

"Religion in America seems tied up with questions of identity in ways that are not the case in other industrialized countries. When you ask Americans about their religious beliefs, it's like asking them whether they are good people, or asking whether they are patriots. They'll say yes, even if they cheated on their taxes, bilked Medicare for unnecessary services, and evaded the draft. Asking people how often they attend church elicits answers about their identity—who people think they are or feel they ought to be, rather than what they actually believe and do." - Eivind