¶ Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths. But I say unto you, Swear not at all; neither by heaven; for it is God’s throne. Nor by the earth; for it is his footstool. Neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
— Matthew 5:33-37
The backstory here was that in the culture of the time, people would preface their words with statements of oaths as to how truthful they were, "By my beard", "by the life of my mother", and so forth. There were elaborate formal progressions as to which ones were "greater" than others, and what public penance one should perform if caught lying (note, not if you do lie, but if you are caught.). The carpenter's son instead said, "All that stuff, stop it. Tell the truth, say what you will do or not do, and leave it at that."
It was part of how the the religion he founded replaced shame (which is based on being caught) with guilt (which is based on knowledge of falling short of internallized morality), and thus helped seed the creation of a working civilization.
This is on my mind, because of recent conversation I had that went something like this:
Me: "It would be going back on my word."
Other: "You gave your word?!"
Me: "I said I would do it."
Other: "For me, 'giving my word' is something more formal."
Me: "For me, it's the same thing."
I fall short sometimes, but it's how I try to be. I always find it disappointing that this is not a shared ethical basic.