Treaty of Tripoli (1797)
Many Christians today try to rewrite history to establish Christianity as foundational to the US beginnings, and cast the founders as adhering to their version of Christianity who promoted religion in government. Most founders were Deists.
A treaty was signed in 1797, when the founders of the USA were living and in government in various capacities. They explicitly setup the US government to be agnostic with regard to religion — a government of, by, and for ALL the people. Unlike European nations, the US would not have an official church. Instead, there would be a 'wall of separation of church and state', as Thomas Jefferson would say. This statement was uncontroversial at the time, passing overwhelmingly.
Art. 11. As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquility, of Mussulmen [Muslims]; and as the said States never entered into any war or act of hostility against any Mahometan [Muslim] nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries.