While I believe this country was founded to not support a mainstream religion, it still gave the freedom to believe in whatever you want. Since the majority religion in America at the time was Christian, it was unavoidable to not pander to them simply to garner support for the regime. So in effect, it was a Chrstian nation by majority, not by choice. It can easily be changed at any time and if enough of the population support it, it will be as such.