Contrary to what some misguided letter writers in the Sun Herald have stated the Founding Fathers never intended for America to be a Christian Nation in fact they plainly stated in the Treaty of Tripoli Article 11 that it was not:
“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 tranquility, 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.”
In fact some of the Founding Fathers were not Christians at all they were Deist. Deism or “the religion of nature” was a form of rational theology that emerged among “freethinking” Europeans in the 17th and 18th centuries. Deists insisted that religious truth should be subject to the authority of human reason. They denied that the Bible was the revealed word of God, rejected scripture as a source of religious doctrine, and rejected all the supernatural elements of Christianity. The doctrines of original sin, the account of creation found in Genesis, and the divinity and resurrection of Christ were castigated as irrational beliefs unworthy of an enlightened age.
Benjamin Franklin (Deist) A Dissertation on Liberty and Necessity, Pleasure and Pain
Ethan Allen (Deist) The Only Oracle of Man
Thomas Paine (Deist) Age of Reason
Thomas Jefferson (Deist)
Even those who espoused a Christian ideology often “were not all that enthusiastic about religion, certainly not about religious enthusiasm.” According to John Adams "This would be the best of all possible worlds, if there were no religion in it.”.
History is Elementary has an excellent post on the subject