The Founders made a very good try. It was the "Age of Reason", and not just because of Paine's book.
I'm not questioning the importance of Christianity, or the fact that many Founders were Christians, or that Jefferson had at times attended church in a government building.
What I am challenging is your assertion that America was founded as a "Christian Nation". It very clearly was founded as a nation that enshrined freedom of religion, including ALL religions, and even those who had no religion.
I never said that. America was founded by Christians who saw faith as essential to maintaining a free republic--"Religious nuts with guns" as P.J. O'Rouke elegantly put it. There was never an intent to drive faith out of the public square or to have an absolute separation of church and state. The debate on the wording of the first amendment is fascinating because they were concerned that someday, someone might read it exactly as it has been read since 1947--that religion must be censored. The founders took great pains to see that didn't happen. But it did.
Oh well...the free exercise of religion lasted for over 150 years. Not too shabby.