Article Two, Section 1, Clause 8:
Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:—”I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”
You will note there is no reference to God here in the Constitution.
A. Lincoln is the first president known to have added the phrase, based on contemporary reporting, at his Second Inaugural.
I do solemnly swear (or affirm) TO WHO??