Here’s how a relativist would administer a judicial oath.
Do you solemnly swear to tell what’s true for you, the whole of what’s true for you, and only what’s true for you?
Would that make you feel a little uneasy if the person being sworn in were testifying against you? Assuming you’re innocent, wouldn’t you rather they just swear to tell the truth?
Update: See further discussion in A pragmatist takes the witness stand.