The Treaty of Tripoli (1797) says:
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 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.
Wow. How awesome is that? And this is from right after the founding, too – within a generation of the founding, so one assumes that the author of the treaty knew what he was talking about. Even if it was written by some random guy, it was signed with the authority of the US government. How awesome is that? More ammo for future arguments!