Guaranteed

Indeed, your servant guaranteed the boy’s safety to my father, saying, ‘If I do not return him to you, I will bear the guilt before you, my father, all my life.’

Therefore, the promise comes by faith, so that it may rest on grace and may be guaranteed to all Abraham’s offspring—not only to those who are of the law, but also to those who are of the faith of Abraham. He is the father of us all.

So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath.