I always like this point from The Treaty of Tripoli back in 1796:

"Article 11: As the Government of the United States of America is not, in any sense, founded on the Christian religion..."

Keep in mind that this treaty (to end the war with the Barbary Pirates) was signed by President John Adams and ratified by the US senate, the majority of whom also signed the Constitution, so when someone says the US was founded as a Christian nation, I always tell them:

"Not according to the founding fathers!"