The following is only part of an excellent article by Jeff Schweitzer at The Huffington Post:
The facts of our history are easy enough to verify. Anybody who ignorantly insists that our nation is founded on Christian ideals need only look at the four most important documents from our early history — the Declaration of Independence, the Articles of Confederation, the Federalist Papers and the Constitution — to disprove that ridiculous religious bias. All four documents unambiguously prove our secular origins.
Declaration of Independence (1776)
The most important assertion in this document is that “to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed.”
Note that the power of government is derived not from any god but from the people. No appeal is made in this document to a god for authority of any kind. In no case are any powers given to religion in the affairs of man.
Remember, too, that this document was not written to form or found a government but was stating intent in a way that was meant to appeal to an audience with European sensibilities. Only four times is there any reference at all to higher powers — “Laws of Nature and of Nature’s God,” “Supreme Judge of the world,” “their Creator,” and “divine Providence” — and in all four cases the references to a higher power appeal to the idea of inherent human dignity, never implying a role for a god in government.
Articles of Confederation (1777)
Throughout the entire document, in all 13 articles, the only reference to anything remotely relating to a god is a term used one time, “Great Governor of the World,” and even then only in the context of general introduction, like “Ladies and gentlemen, members of the court....” Unlike the Declaration of Independence, this document did indeed seek to create a type of government in the form of a confederation of independent states. The authors gave no power or authority to religion. And this document is our first glimpse into the separation of church and state, because just as the Articles of Confederation give no authority to religion in civil matters, so too does the document deny any authority of government in matters of faith.
U.S. Constitution (1787)
This one is easy, because the Constitution of the United States of America makes zero reference to a god or Christianity.
The only reference to religion, found in Article VI, is a negative one: “[N]o religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.” And of course we have the First Amendment, which states that “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof.”
Federalist Papers (1787-88)
While Thomas Jefferson was the genius behind the Declaration of Independence, John Jay, Alexander Hamilton and James Madison (publishing under the pseudonym “Publius”) were the brains providing the intellectual foundation of our Constitution. And what brilliance they brought to the task. The first time I picked up the Federalist Papers, I intended to scan the book briefly and then move on to more interesting pursuits. But I could not put it down; the book reads like an intriguing mystery novel with an intricate plot and complex characters acting on every human emotion. There is no better way to get into the minds of our founding fathers and understand their original intent than by reading this collection of amazing essays.
As with the Constitution, at no time is a god ever mentioned in the Federalist Papers. At no time is Christianity every mentioned. Religion is only discussed in the context of keeping matters of faith separate from concerns of governance, and of keeping religion free from government interference.
The founding fathers could not be clearer on this point: God has no role in government; Christianity has no role in government. They make this point explicitly, repeatedly, in multiple founding documents. We are not a Christian nation.