swear an oath
take an oath

I think, for having given your word or sworn an oath.

When someone has to give evidence in a court of law they "swear an oath".

"Speak the name of the man to whom you have sworn your oath."

"Take it in your right hand now and swear an oath."

She swore an oath to play her game by the rules.

I swore an oath on that one a long time ago.

I swore an oath to die rather than let that happen.

He swore a round oath, and looked at us, from one to the other.

I have sworn a holy oath to drive you and your children from the house.

To this end, he has sworn an oath to their total destruction.