Rule 603 states that the oath is 'calculated to awaken the witness' conscience and impress the witness' mind with the duty' to tell the truth.

CNN: Sen. Allard's closed-door impeachment statement

Testimony before a federal grand jury is given under oath, with the witness swearing to 'tell the truth, the whole truth, and nothing but the truth.

CNN: Sen. Allard's closed-door impeachment statement

And right now, we are bearing witness to the Iranian people's belief in that truth, and we will continue to bear witness.

CNN: Obama to Iran: 'The whole world is watching'

That is the fundamental truth to which Canadians and Americans have borne witness for almost two centuries.

WHITEHOUSE: President Obama and Prime Minister Harper Press Conference | The White House