It is important to note that what makes a nation "Christian" is not what religion the majority of people profess or the behavior they exhibit.
What makes a "Christian nation" is the foundation of the nation's government, the basis for the laws. It is impossible to expunge every morsel of the Biblical underpinnings that form the foundation of America and keep it America.
We are a Christian nation...whether people like it or not. From our three branches of government (Isaiah 33:22) to our levels of government (Jethro instructing Moses to divide oversight into groups), our laws and government are Biblical.
Just look at this list! http://www.faithfacts.org/christ-and-the-culture/the-bible-and-government
To make it anything else, this Constitutional Republic would have to be scrapped and a completely different form of government instituted. (In fact, this is what many are indeed trying to do because of their disdain for and rejection of God Almighty.)
This leads to the idea of "American Exceptionalism." The Left has intentionally perverted this term to make citizens believe that it means we think we are better than everyone else on the planet. Rather, it refers to the fact that this was the first nation *ever in history* - an exception - that was founded on the idea of Christian self-government, that man did not need a ruler, but could govern himself.
Since America's founding and her subsequent never-seen-before freedom, other nations have taken note of the great prosperity and generosity self-government affords and have adopted similar forms of government. But now in America, as man voluntarily cedes more and more control of himself over to man (via the civil government), we are seeing freedoms constrict and prosperity waning.
This is the natural law put in place by God: As self-government increases, civil government decreases. As self-government decreases, civil government increases.
Man who is not controlled by Christ will ALWAYS seek control by man in the form of increased laws, regulations, and restrictions.
Remember Rome? They had a law for everything. And I seem to recall...they didn't last.