The requirement is constitutional. What’s the constitution say?

https://www.senate.gov/about/origins-foundations/senate-and-constitution/oath-of-office.htm

https://constitution.congress.gov/browse/article-6/clause-3/

Article VI Supreme Law

Clause 3 Oaths of Office 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.

ArtVI.C3.1 Oaths of Office Generally ArtVI.C3.2 Religious Test ArtVI.C3.2.1 Historical Background on Religious Test for Government Offices ArtVI.C3.2.2 Interpretation of Religious Test Clause