People always use the name of someone greater than themselves to make a promise with an oath. The oath proves that what they say is true, and there is no more arguing about it.
David asked the Egyptian, “Will you lead me to the people who took our families?” The Egyptian answered, “If you make a special promise before God, I will help you find them. But you must promise that you will not kill me or give me back to my master.”
Saul’s grandson, Mephibosheth, came down to meet King David. Mephibosheth had not cared for his feet, trimmed his mustache, or washed his clothes since the day the king left Jerusalem.
But the king answered, “What can I do, sons of Zeruiah? Yes, Shimei is cursing me, but the Lord told him to curse me. And who can ask him why he did that?”