Wash Park Prophet concludes we are a Christian nation only in Gallup polls. The entire entry is worth a read; he ends with this:
Most Americans aren’t really very different, even though we aren’t as trained to recognize it as such. Many Japanese parents teach their children Confucian proverbs. Americans are as likely to offer their children moral guidance form Aesop and the Brothers Grimm and Winnie the Pooh, as they are from the Proverbs or the Book of Job.
This is why there will never be a full fledged theocracy in the United States. While Christianity has made some narrow inroads into the American mythology, for example in the Pledge of Allegiance and the “In God We Trust” motto, which were themselves bad decisions, the Christian majority is illusory. Many nominal Christans in America don’t believe in Noah much more than they do in Batman, and many people who give doctrinaire answers on the phone to pollsters asking about the miracles of Christianity and the Creation story, are about as sincere as a parent asked by a child about Santa Claus. Decorum and good breeding dictate a certain answer, but that answer isn’t always sincere when the truth really counts.