The Declaration of Independence is *not* the founding document. The U.S. Constitution is, and it is based on British Common Law, which existed for hundreds of years before christianity was "introduced" to Britain.
Even if all of the founders where christians (they weren't), they created a Constitution which keeps church and state separate, to protect both, but mainly to prevent religious tyranny.
The only mentions of religion in the U.S. Constitution are negative: no religious test shall be required to hold public office, and the government shall not establish a religion.
The Treaty of Tripoli states that the U.S. was *not* in any way founded on christianity.
Learn history from reliable sources, not Answers in Genesis.
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Let some of the founders speak for themselves:
"No man [should] be compelled to frequent or support any religious worship, place, or ministry whatsoever, nor [should he] be enforced, restrained, molested, or burthened in his body or goods, nor ... otherwise suffer on account of his religious opinions or belief ... All men [should] be free to profess and by argument to maintain their opinions in matters of religion, and ... the same [should] in no wise diminish, enlarge, or affect their civil capacities." - Thomas Jefferson, Statute for Religious Freedom, 1779
"I have examined all the known superstitions of the word, and I do not find in our particular superstition of Christianity one redeeming feature. They are all alike founded on fables and mythology. Millions of innocent men, women and children, since the introduction of Christianity, have been burnt, tortured, fined and imprisoned. What has been the effect of this coercion? To make one half the world fools and the other half hypocrites; to support roguery and error all over the earth." - Thomas Jefferson; letter to William Short
"The clergy, by getting themselves established by law and ingrafted into the machine of government, have been a very formidable engine against the civil and religious rights of man." - Thomas Jefferson
"Millions of innocent men, women and children, since the introduction of Christianity, have been burnt, tortured, fined and imprisoned; yet we have not advanced one inch towards uniformity." - Thomas Jefferson; Notes on Virginia, 1782
"But it does me no injury for my neighbor to say there are twenty gods or no God. It neither picks my pocket nor breaks my leg." - Thomas Jefferson
"Where the preamble declares, that coercion is a departure from the plan of the holy author of our religion, an amendment was proposed by inserting "Jesus Christ," so that it would read "A departure from the plan of Jesus Christ, the holy author of our religion;" the insertion was rejected by the great majority, in proof that they meant to comprehend, within the mantle of its protection, the Jew and the Gentile, the Christian and Mohammedan, the Hindoo and Infidel of every denomination." - Thomas Jefferson, Autobiography, referring to the Virginia Act for Religious Freedom
"I never submitted the whole system of my opinions to the creed of any party of men whatever in religion, in philosophy, in politics, or in anything else where I was capable of thinking for myself. Such an addiction is the last degradation of a free and moral agent." - Thomas Jefferson, letter to Francis Hopkinson, March 13, 1789
"They [the clergy] believe that any portion of power confided to me, will be exerted in opposition to their schemes. And they believe rightly; for I have sworn upon the altar of god, eternal hostility against every form of tyranny over the mind of man. But this is all they have to fear from me: and enough, too, in their opinion." - Thomas Jefferson, letter to Dr. Benjamin Rush, September 23, 1800
"Believing with you that religion is a matter which lies solely between man and his God, that he owes account to none other for his faith or his worship, that the legislative powers of government reach actions only, and not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof,' thus building a wall of separation between church and State." - Thomas Jefferson, letter to Danbury Baptist Association, CT, January 1, 1802
"History, I believe, furnishes no example of a priest-ridden people
maintaining a free civil government. This marks the lowest grade of ignorance of which their civil as well as religious leaders will always avail themselves for their own purposes." - Thomas Jefferson, letter to Alexander von Humboldt, December 6, 1813
"Christianity neither is, nor ever was a part of the common law." -
Thomas Jefferson, letter to Dr Thomas Cooper, February 10, 1814
"In every country and every age, the priest has been hostile to liberty. He is always in alliance with the despot, abetting his abuses in return for protection to his own. It is easier to acquire wealth and power by this combination than by deserving them …" - Thomas Jefferson, letter to Horatio G Spafford, March 17, 1814
"Man once surrendering his reason, has no remaining guard against absurdities the most monstrous, and like a ship without rudder, is the sport of every wind." - Thomas Jefferson, letter to James Smith, 1822
"And the day will come when the mystical generation of Jesus, by the supreme being as his father in the womb of a virgin will be classed with the fable of the generation of Minerva in the brain of Jupiter. But may we hope that the dawn of reason and freedom of thought in these United States will do away with this artificial scaffolding, and restore to us the primitive and genuine doctrines of this most venerated reformer of human errors." - Thomas Jefferson
"It is between fifty and sixty years since I read [the Book of
Revelation], and I then considered it merely the ravings of a maniac, no more worthy nor capable of explanation than the incoherences of our own nightly dreams." - Thomas Jefferson, letter to General Alexander Smyth, January 17, 1825
"What influence in fact have Christian ecclesiastical establishments had on civil society? In many instances they have been upholding the thrones of political tyranny. In no instance have they been seen as the guardians of the liberties of the people. Rulers who wished to subvert the public liberty have found in the clergy convenient auxiliaries. A just government, instituted to secure and perpetuate liberty, does not need the clergy." - James Madison, Fourth US President
"Religion and government will both exist in greater purity, the less they are mixed together." - James Madison, Fourth US President
"During almost fifteen centuries has the legal establishment of
Christianity been on trial. What have been its fruits? More or less in all places, pride and indolence in the Clergy, ignorance and servility in the laity, in both, superstition, bigotry and persecution." - James Madison; A Biography in his Own Words, edited by Joseph Gardner, p. 93
"All national institutions of churches, whether Jewish, Christian, or Turkish, appear to me no other than human inventions, set up to terrify and enslave mankind, and monopolize power and profit." - Thomas Paine
"It is error alone which needs the support of government. Truth can stand by itself." - Thomas Jefferson (1743-1826), Third US President
"It behooves every man who values liberty of conscience for himself, to resist invasions of it in the case of others: or their case may, by change of circumstances, become his own." - Thomas Jefferson (1743-1826), Third US President