The Treaty of Tripoli, signed by president John Adams, said this in 1797:
"the Government of the United States of America is not, in any sense, founded on the Christian religion..."
There's not much room for modern-day believers to hope they can spin that into an idea that America was a Christian nation at the very start.
If it was a Christian nation, then there'd be no room here for a wide variety of people with different ideas like Muhammad Ali and Kareem Abdul-Jabbar (Muslim), George Harrison and Julia Roberts (Hinduism) and an endless list of Jews who made huge contributions to American society in spite of early prejudice and resistance to their very presence on our shores.