When people ask whether this is a "Christian Nation" or state that it is they may have a tentative point. This is assuming that what they mean is that the majority of believers in the US happen to be Christian. That is true. The majority of religious citizens do identify as Christian. However, even this perspective on such a statement is highly misleading. Christianity, like all religions, is not unified. As I have pointed out before ("Shepherd of a Wayward Flock" March 12, 2012) there are at last count 33,000+ identifiable denominations/sects within Christianity. I fail to see how the phrase "Christian Nation" is useful as a description of our country.
Then there is the problem that what many really mean by this phrase is not related to comparative prevalence of Christianity in the US. Many are trying to assert that our country was founded through, by, and for Christianity. In this case the answer is a resounding NO. Christianity had little to do with our founding as a country unless you mean as a negative example. Many of the founding fathers, though religious themselves*, had seen first hand what terrible things can happen when you mix religion into politics. We were, in point of fact, intentionally founded as a secular nation. It is not an accident that there isn't a single reference to God or Christianity in the constitution any more than it is intentional that there are only two reference to religion in general. Both negate the notion of a Christian founding. Article VI includes, "...but no religious test shall ever be required as a qualification to any office or public trust under the United States." And, of course, there is the 1st Amendment which does establish the essential principle of separation of church and state.
Simply having a majority of Christians does not alter the nature of our government or change our history. We can only be said to be a "Christian Nation" in the most narrow and superficial sense of the phrase.
Thinking Atheist recently put out a pretty good video that addresses this topic:
*Many were Deists. Most who like to talk about the founding father's faith or the country being Christian would be horrified at what the deists of the time thought about God and religion.