The United States of America is a child of the Enlightenment, and an understanding of this fact is essential to understanding what we are, what we are not, and for what we stand.  What we are NOT is a christian nation,  and we were not founded on christian principles  (whatever that may mean).

What is the Enlightenment? “The Enlightenment has been defined in many different ways, but at its broadest was a philosophical, intellectual and cultural movement of the seventeenth and eighteenth centuries. It stressed reason, logic, criticism and freedom of thought over dogma, blind faith and superstition. Logic wasn’t a new invention, having been used by the ancient Greeks, but it was now included in a worldview which argued that empirical observation and the examination of human life could reveal the truth behind human society and self, as well as the universe. All were deemed to be rational and understandable. The Enlightenment held that there could be a science of man, and that the history of mankind was one of progress, which could be continued with the right thinking.

Consequently, the Enlightenment also argued that human life and character could be improved through the use of education and reason. The mechanistic universe – that is to say, the universe when considered to be a functioning machine – could also be altered. The Enlightenment thus brought interested thinkers into direct conflict with the political and religious establishment; these thinkers have even been described as intellectual “terrorists” against the norm. They challenged religion with the scientific method, often instead favouring deism. The Enlightenment thinkers wanted to do more than understand, they wanted to change for, as they believed, the better: they thought reason and science would improve lives…The scientific revolution of the seventeenth century shattered old systems of thinking, and allowed new ones to emerge. The teachings of the church and Bible, as well as the works of classical antiquity so beloved of the Renaissance, were suddenly found lacking when dealing with scientific developments…In general, Enlightenment thinkers argued for freedom of thought, religion and politics. The philosophes were largely critical of Europe’s absolutist rulers…The philosophes were deeply critical, indeed even openly hostile, to the organised religions of Europe, especially the Catholic Church whose priests, pope and practices came in for severe criticism…The Enlightenment affected many areas of human existence, including politics; perhaps the most famous examples of the latter are the US Declaration of Independence and the French Declaration of the Rights of Man and the Citizen… The Enlightenment era saw a general turn away from the dominance of the church and the supernatural, with a reduction in belief in the occult, literal interpretations of the Bible and the emergence of a largely secular public culture, and a secular “intelligentsia” able to challenge the previously dominant clergy.”

see: http://europeanhistory.about.com/od/thenineteenthcentury/a/enlightenment.htm

The founding fathers were enlightened, and the founding mother was the Enlightenment.  One can find words and ideas from the  Enlightenment  in our declaration, but none from christianity, which is why we need to set aside a day each year where we pay homage to our true heritage.