Those that serve us in Congress, in both the House and the Senate, have to take this oath of office. This is another indicator of the importance of the Constitution of the United States of America and the principles that the Founding Fathers believed in and that provides the foundation of the Constitution.
“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.”
— U.S. Constitution, Article VI, clause 3
As we see in this quote from the constitution, we see that this requirement is constitutionally mandated. It is so not only for Congress but for State Legislatures and all executive and judicial officers. This oath is not a religious test as the constitution expressly prohibits that.