All officers, whether elected or appointed, shall, before entering upon the duties of their respective offices, take and subscribe the following oath or affirmation:
“I do solemnly swear (or affirm, as the case may be) that I will support the Constitution of the United States and the Constitution of the State of Illinois, and that I will faithfully discharge the duties of the office of according to the best of my ability.”
This oath or affirmation so subscribed shall be filed in the office of the Clerk.
(ILCS Ch. 65, Act 5 § 3-14-3) (‘69 Code, § 2-82)