We still have under god in the pledge. We still have in god we trust on our money. It's only been a few years since we removed prayer from school, and many people are fighting to put it back. We have laws against gays that are 100% based in faith and aren't backed up by any true morals.
Obviously we are not a nation of Christians because of the fact that immigrants come from all over the world and come to the United States and with them comes their religion. They will past that down to others and that's how religions spread. Our country believes mainly in the Christian religion but we allow people with varying religions to come to the United States and practice their religion.
This is kind of an odd question, can any nation be Christian? The Bible states that it is not by works that we are saved, but by accepting the very gift of salvation thru Jesus Christ. The issue at hand is that this is a personnal decision, not one I can make for anyone else. For a nation to be Christian you would have to force people to make this decision which than is not a choice.