I do not look at America as a Christian nation,but it is a nation that allows all religions to be practiced here,even satanism.It only appears that way because the majority at the founding of the country,if not all,were claiming Christianity,but I do not believe the founding fathers believed the Bible according to truth.
A nation that is a Christian nation would be like Israel in the Old Testament,when they obeyed God,when God told them to not allow any other religion for it is false and would lead them astray from the one true God.
America would be no better than Babylon when king Nebuchadnezzar acknowledged the God of Israel as a God of gods,but he believed in other gods,and astrology,and such things.
Even if America says that the God of Israel is the greatest,they still allow all religions to be practiced there,even occultism,witchcraft,and such things.