Article 11 of the US’s Treaty with Tripoli reads:
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 tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan 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.
The treaty was “authored by American diplomat Joel Barlow in 1796, [then] sent to the floor of the Senate, June 7, 1797, where it was read aloud in its entirety and unanimously approved. John Adams, having seen the treaty, signed it and proudly proclaimed it to the Nation.” Today, claims that the US is a Christian nation, founded on Christian principles are common. In contrast, that statement was accepted without protest in 1797 according to this article. Since the US was actually not founded on any Christian principles whatsoever, that’s just what I’d expect. Still, it’s lovely to see it said explicitly by the Founders themselves.