I argued in Was America Founded as? a Christian Nation? that most Americans, prior to the 1980, believed that they were living in a Christian nation.
Over at American Creation, Tom Van Dyke offers some more evidence to back up that assertion:
I don’t think we put enough stress on the necessity of implanting in the child’s mind the moral code under which we live. The fundamental basis of this Nation’s law was given to Moses on the Mount. The fundamental basis of our Bill of Rights comes from the teachings which we get from Exodus and St. Matthew, from Isaiah and St. Paul. I don’t think we emphasize that enough these days. If we don’t have the proper fundamental moral background, we will finally wind up with a totalitarian government which does not believe in rights for anybody except the state.
Harry S Truman
February 15, 1950