The catholic encyclopedia will tell you why.
And there you go…..
U.S. Code › Title 28 › Part I › Chapter 21 › § 453
28 U.S. Code § 453 - Oaths of justices and judges
Current through Pub. L. 114-19. (See Public Laws for the current Congress.)
• US Code
• Notes
Each justice or judge of the United States shall take the following oath or affirmation before performing the duties of his office: “I, XXX XXX, do solemnly swear (or affirm) that I will administer justice without respect to persons, and do equal right to the poor and to the rich, and that I will faithfully and impartially discharge and perform all the duties incumbent upon me as XXX under the Constitution and laws of the United States.
"So help me God.”
https://www.law.cornell.edu/uscode/text/28/453