To take an oath, is to swear solemnly; to vow.
Example: "I, do solemnly swear that I will support and defend the Constitution of the United States against all enemies, foreign and domestic."
See Pledge of Allegiance.
U.S. Constitution Article VI, Section 3, requires the following:
|“||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.||”|