In his book on Histories and Fallacies Carl Trueman has this to say about America as a Christian nation.  In context he is discussing the Genetic Fallacy.  A great example of this, writes Trueman, is the myth of America as a Christian nation:

Many readers might well be thinking at this point that this fallacy is so obviously problematic that it cannot possibly have much force within the writing of history today.  True, its flaws are obvious; but, in fact, it does enjoy considerable vogue in some quarters.  Take, for example, the most radical wings of the Christian America movement where the argument is that America was founded by men motivated by and large by their commitment to the Christian faith and their desire to build a Christian nation.  Thus, America was and is—or at least, ought to be—a Christian nation, and her founding documents embody Christian virtues.  This leads to interpretations of the present that can engage simply in anachronistic value judgments on actions and events; or, perhaps in a more sinister way, connect America to events in biblical prophecy, God’s providential plans for the world, etc.  (Trueman, Histories and Fallacies, p159)

I may be reviewing this book later this week, but I’m not sure yet since it’s probably only meant for history nerds like myself.  But if you are a fan of history, or are considering writing history, this book is worthy of purchasing.  You can get the E-book for only 7.99