I do not like the reappearance of the Jesuits. … Shall we not have regular swarms of them here, in as many disguises as only a king of the gipsies can assume, dressed as printers, publishers, writers and schoolmasters? If ever there was a body of men who merited damnation on earth and in Hell, it is this society of Loyola’s. Nevertheless, we are compelled by our system of religious toleration to offer them an asylum.
— John Adams, letter to Thomas Jefferson, May 5, 1816
Cabalistic Christianity, which is Catholic Christianity, and which has prevailed for 1,500 years, has received a mortal wound, of which the monster must finally die. Yet so strong is his constitution, that he may endure for centuries before he expires.
— John Adams, letter to Thomas Jefferson, July 16, 1814.
Indeed, Mr. Jefferson, what could be invented to debase the ancient Christianism which Greeks, Romans, Hebrews and Christian factions, above all the Catholics, have not fraudulently imposed upon the public? Miracles after miracles have rolled down in torrents.
— John Adams, letter to Thomas Jefferson, December 3, 1813.
I cribbed the quotes from Positive Atheism, which has many more. It’s a good place to stock up on ammo for your next encounter with a conservative who insists the US was intended to be a Christian country.