Article VI, Clause 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.
Congress may require no other oath of fidelity to the Constitution, but it may add to this oath such other oath of office as its wisdom may require. 1 It may not, however, prescribe a test oath as a qualification for holding office, such an act being in effect an ex post facto law, 2 and the same rule holds in the case of the states. 3