The USA is not a Christian nation/state

   "Is it true that George Washington said that the United States is not
   in any sense founded upon the Christian religion?" 
   
   No. The quotation often given is in fact from Article XI of the 1797
   Treaty of Tripoli (8 Stat 154, Treaty Series 358):
   
     Article 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 or enmity against the laws, religion or tranquility of
     Musselmen, -- and as the said States never have entered into any war
     or act of hostility against any Mehomitan 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 text may be found in the Congressional Record or in treaty
   collections such as Charles Bevans' "Treaties and Other International
   Agreements of the United States of America 1776-1949", vol. 11 (pp.
   1070-1080).
   
   The English text of the Treaty of Tripoli was approved by the U.S.
   Senate on June 7, 1797 and ratified by President John Adams on June
   10, 1797. It was recently discovered that the Arabic version of the
   treaty not only lacks the quotation, it lacks Article XI altogether.
   
   The person who translated the Arabic to English was Joel Barlow,
   Consul General at Algiers, a close friend of Thomas Paine -- and an
   opponent of Christianity. It is possible that Barlow made up Article
   XI, but since there is no Arabic version of that article to be found,
   it's hard to say.
   
   In 1806 a new Treaty of Tripoli was ratified which no longer contained
   the quotation.