Was America founded as a Christian nation?

Short answer - no, the religious fundamentalist would disown many of the founding fathers and less than 10% of families at the time of the Revolution were members of congregations.

The official answer to the question is provided by the Treaty of Tripoli in 1796-1797. "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 Musselmen..." This was debated and signed by the Senate with no objection to the clause.

The religious language in the Declaration of Independence is taken from Deism and Freemasonry. Both of these hold to a Supreme Being or Power but do not profess belief in the Trinity, the Divinity of Jesus (George Washington never mentions Jesus in any of thousands of letters and speeches), or in Hell.

Many fundamentalist who are not particularly well educated and have a shaky grasp of history and different religions simply take God to refer to the Christian God and their associated concepts. They are mistaken on this.

The word God did not appear on currency until after the Civil War and the first major reference to Christianity in any U.S. government document occurred in 1892 when one Supreme Court Judge declared the United States a Christian nation but specified that was a personal opinion only and was not a legal pronouncement.

The Constitution specifies that "Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof" and "no religious test shall ever be required as a qualification to any office or public trust under the United States."

The Pledge of Allegiance was not created until 1892 and the phrase "under God" was added in 1954. The pledge is somewhat silly anyway, why are you pledging to a flag? If you pledge to anything, all but one other country doesn't have a pledge, shouldn't it be to the Constitution? Office holders have to swear to uphold the Constitution. Under our system of government you should not swear to any leader.

This page here by Jim Walker has more good information but has an annoying javascript that I couldn't shut off.