US Constitution, Article VI, paragraph 3:

The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious test shall ever be required as a qualification to any office or public trust under the United States.

You would have thought if they meant Christianity to supersede other religions, they would not have included this bit.