Article VI, Section 3

This reinforces the Supremacy Clause; that all public officials respect the Constitution of the United States foremost above all. In addition, no religious test may be imposed as a qualification to hold any office of the government. The oath it peaks of is the one that all members of Congress take:

I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion, and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me G-d.