This is a good article. It mentions many familiar names.
A taste:
Gregg Frazer is dean of humanities and professor of history and political studies at the Master’s University in Santa Clarita, Calif. In his 2012 book, The Religious Beliefs of America’s Founders, he examined the private writings of eight Founders: John Adams, Thomas Jefferson, Benjamin Franklin, Gouverneur Morris, James Wilson, James Madison, Alexander Hamilton, and George Washington. He says personal diaries and correspondence—not church affiliation or public comments—reveal the men’s most candid, reliable thoughts about what they believed. And it wasn’t Biblical Christianity, he says, even though they believed in God, often attended church, and used Christian-like terminology and images in public statements and documents.
Frazer found, for instance, that many of these founders (including those most involved in writing our founding documents) rejected the deity of Christ and the Trinity yet believed God intervened in man’s affairs. Frazer says their differing personal beliefs were a blend of Christianity, natural law, and secular reason, with secular reason trumping all.
The debate over the founders’ faith has divided academia for decades, and not everyone agrees with Frazer’s assessment. For example, in their 2006 book George Washington’s Sacred Fire, historian Peter Lillback and writer Jerry Newcombe argued America’s first president was “an orthodox, Trinity-affirming believer in Jesus Christ,” though not an “evangelical” in the modern sense. Other founders, such as Patrick Henry and Samuel Adams, are widely recognized to have held orthodox Christian beliefs.
Whatever the founders’ personal beliefs, Frazer says they thought religion should be encouraged because it exhorts people to be moral, upright citizens—the necessary ingredient for a republic to work. But Article 6 of the Constitution states no religious test may be required for public office, and the First Amendment prohibits Congress from establishing any religion and from making laws to bar religious free exercise. Moreover, the Treaty of Tripoli—unanimously signed by the Senate and three founders in 1797—pointedly says America is not a Christian nation.
Frazer says when Christian nationalists say God will only bless America if America is Christian, he wants to ask them about ancient civilizations that thrived without honoring God: “How do you explain Sparta? Sparta lasted 800 years, and it was perhaps the most ungodly regime in history.”