In 1802 President Thomas Jefferson wrote in a letter to the Baptists of Danbury, Connecticut that the First Amemdment to the US Constitution builds a wall of separation between Church and State.
In 1797 the Treaty of Tripoli stated that the US government is in no sense based on the Christian religion. This treaty was debated and passed by the Senate. It was then signed by President John Adams.
These men were the Founding Fathers. That is proof that Christians are mistaken about the origins of US law.