In fact, here is a great quote dug up by Andrew Sullivan from The Conservative Soul:
In 1797, the U.S. Senate unanimously approved the "Treaty of Tripoli," an attempt to deal with Muslim piracy and terrorism in the Mediterranean. One of its clauses read:
"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."
It is hard to think of a leading contemporary Republican insisting that American government "is not, in any sense, founded on the Christian religion." In the early republic, not a single senator dissented.