James Madison.

Too many people seem to think the Founders wanted the Dominionist idea of a Christian America. As the majority of reliable quotes emphasizes, they wanted a secular government as the strongest guarantor of religious freedom for all. Indeed, if the Founders had wanted the United States to be a Christian nation the way Saudi Arabia is a Salafi* Sunni Islamic nation, they would have written the Constitution that way in the first place!

*I added the qualifiers because the Kingdom of Saudi Arabia is as intolerant of nonconforming Muslims as it is of Khafirs (infidels), and the Salafi (aka Wahabi) version of Islam is an ultra-conservative, very right-wing sect that brooks no competition from Shiites, liberal Muslims, and outright infidels.