We could start at the forefathers and work up.. Here's some great reading that walks the reader through how the drafters of our Constitution based their ideology on Enlightenment http://wiki.answers.com/Q/What_did_religion_mean_to_our_founding_forefathers
Jefferson and Franklin in particular were critical of organized religion.
Jefferson quotes regarding religion:
http://nobeliefs.com/jefferson.htm
Benjamin Franklin on religion:
http://exchristian.net/exchristian/2002/03/ben-franklin-quotes.php
Throughout the years since - we have put so much emphasis on diversity on openness to all races, creeds, religions and belief systems that our American identity our unique culture is wholly different than any of the original countries that any of us came from. Limiting this nations faith to strictly Christianity is naive. This would not be America otherwise - it would be another Iran or Saudi Arabia (minus muhammad of course..)
Separation of church and state:
http://en.wikipedia.org/wiki/Separation_of_church_and_state
Religion in America:
http://en.wikipedia.org/wiki/Religion_in_the_United_States
Religious diversity in America:
http://www.religioustolerance.org/chr_prac2.htm
Religious tolerance:
http://www.america.gov/st/texttrans-english/2008/February/20080205173317wltsruh0.2710382.html
Only direct mention of religion in the Constitution:
http://www.usconstitution.net/consttop_reli.html#original
Otherwise lack of religious specifics in Constitution:
http://atheism.about.com/od/godlessliberals/p/Constitution.htm
Bottom line is - while there are millions of American christian families - there is no "exclusively" Christian nation found here.