US is a Christian nation

Overview
It is often argued, especially by Biblical fundamentalists, that the 🇺🇸 is a Christian nation – that is, that one or both of the following are true:
 * The country's laws (especially the Constitution) are based on Christian teachings, especially the Ten Commandments.
 * The founding fathers intended to endorse and support Christianity, or Christian principles, by codifying them in law.

Conclusions
There does not seem to be any truth to these claims, and considerable evidence against:
 * The Constitution codifies a principle known as separation of church and state, specifically stating that "Congress shall make no law respecting an establishment of religion" (Article I)
 * The Constitution specifically prohibits requirements of religious affiliation for holding public office in the US (Article III)
 * Some states, such as North Carolina, do nonetheless make such requirements; this conflict has yet to be addressed.
 * "In God We Trust" on US money and the phrase "under God" in the US Pledge of Allegiance were not part of the original design of the US and were not officially adopted until the 20th century, almost two centuries after the US was founded.

Possible Common Ground
One statement which those on both sides might be able to agree on is "The United States was founded on Judeo-Christian principles." (see for an example of this claim.)

Some problems with this statement:
 * It implies that those principles were adopted because of their Judeo-Christian origins, i.e. the founders were Christians and therefore sought to promote their native ideology as the best ideology to use for the basis of the new country – rather than choosing those principles, without regard to origin, solely on the basis of merit.
 * It implies that Judeo-Christian religious thought should get credit for originating the ideas upon which the US was founded.
 * It implies that Judeo-Christian religious thought (as embodied, perhaps, in the Bible, or at least the Old Testament) is somehow the "real" law, of which the US Constitution is just a sort of representation, excerpt, or set of practical guidelines for application.

Also, as far as I know, nowhere in the Bible is the idea of democracy discussed, nor any of the ideas which are essential parts of democracy (freedom of speech, freedom of the press – the press hadn't been invented in Biblical times anyhow, separation of powers including separation of church and state, and so on). So Judeo-Christianity can hardly take credit for those, though perhaps it may be argued that all of those ideas are somehow derived from Judeo-Christian principles such as "love thy neighbor"; we'd need to see some specifics, however, in order to accept this.

So: if this claim is to be taken seriously, instead of saying "...Judeo-Christian principles", it should name the principles involved – with which Judeo-Christianity can then duly credited as contributions towards the betterment of civilization.

The Constitution
In the 🇺🇸, the separation of church and state is spelled out in the First Amendment to the United States Constitution, which prohibits the Federal legislature from making laws that:
 * Establish a state religion or prefer certain religion (the "Establishment Clause");
 * Prohibit free exercise of religion (the "Free Exercise Clause");
 * Infringe the freedom of speech;
 * Infringe the freedom of the press;
 * Limit the right to assemble peaceably;
 * Limit the right to petition the government for a redress of grievances.

There are some areas where such separation is apparently ignored by tradition:
 * By law, the country's currency now carries the motto "In God We Trust." (Need source: what law?)
 * Congress begins its sessions with a prayer.
 * The words "under God" were, by law, added to the customary United States Pledge of Allegiance (on Flag Day in 1954).

There is obviously a vague area where "free excercise of religion" and "freedom of speech" come into contact with preventing the state from establishing or sanctioning a religion: a state-salaried teacher or principal (for example) arguably represents the voice of the state, which must be restrained by the establishment clause, but the same person is also an individual and has the rights of free speech and free excercise.

Treaty of Tripoli
The Treaty with Tripoli states in part:

As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan nation, it is declared by the parties, that no pretext arising from religious opinions, shall ever produce an interruption of the harmony existing between the two countries.

Related Pages

 * to be the basis of modern law (or of the US Constitution), though the connection seems tenuous at best.

Articles

 * Snopes checks the claim that "Religious symbols and references abound in U.S. capital buildings and the words of America's founders", and finds it mostly false or misleading.
 * Is America a Christian Nation?: supposedly a dialogue between Rev. Dr. Welton Gaddy and Rev. Barry W. Lynn, although both seem to agree on pretty much everything including the necessity of separation of church and state. Significant events in the history of religion in the 🇺🇸 are discussed.

Organizations

 * Americans United for Separation of Church and State

Commentary

 * 2007-10-18 Is America a Christian Nation? mainly addresses some significant side issues, but makes the claim "There is no question that this nation was founded on Jewish-Christian principles."
 * 2007-10-03 Is America a Christian Nation?: "According to John Fea, writing for the History News Network, both the left and the right often answer this question the wrong way."
 * 2007-09-24 Chuck Norris Rewrites US History by Bruce Wilson

Humor

 * 2005-04-06 I Drew This: Thomas Jefferson (longer, color version from 2007-05-01)

Quotes
Kudos to Anonymous on Contrary Brin for some of these quotes

Founding Fathers

 * "This would be the best of all possible worlds, if there were no religion in it." â€” John Adams
 * "Ecclesiastical establishments tend to great ignorance and corruption, all of which facilitate the execution of mischievous projects." â€” James Madison
 * "I do not believe in the creed professed by the Jewish Church, by the Roman Church, by the Greek Church, by the Turkish Church, by the Protestant Church, nor by any Church that I know of. My own mind is my own Church. Each of those churches accuse the other of unbelief; and for my own part, I disbelieve them all." â€” Thomas Paine

Thomas Jefferson

 * "Millions of innocent men, women and children, since the introduction of Christianity, have been burnt, tortured, fined, imprisoned; yet we have not advanced an inch towards uniformity. What has been the effect of coercion? To make one half the world fools, and the other half hypocrites. To support roguery and error all over the earth."
 * "I do not find in orthodox Christianity one redeeming feature."

Benjamin Franklin

 * "When a religion is good, I conceive it will support itself; and when it does not support itself so that its professors are obliged to call for the help of the civil power, 'tis a sign, I apprehend, of its being a bad one."
 * "The way to see by faith is to shut the eye of reason." – Poor Richard's Almanack, 1758