How many Christians are there in America?

This OT for the newsgroup but considering the collective
knowledge and wisdom of the people here, I thought this is the best
place to clarify something that has been confusing me for years. Most
people here know that I am a Jew and an American. Growing up, I always
assumed that most of the non-Jews in America were Christian. Then I
learned that many Americans consider themselves to be agnostics,
athiests, Hindus, Muslims, Buddhists etc. Still, most Americans were
Christian. However, the way some Christian Americans depict
themselves, you would think that only a minority of Americans are
Christians and the rest of the country is filled with heathens. I
point to Evangelical Christians as an example of this. Now are they
just using some definition of Christian that I am unaware of or is
something else going on?