The current oath was enacted in 1884:
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 God.
This is pretty straightforward and the oath for the first Congress of 1789 was simply
“I do solemnly swear (or affirm) that I will support the Constitution of the United States.”
