For thy servant became surety for the lad unto my father, saying, If I bring him not unto thee, then I shall bear the blame to my father for ever.

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 it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,

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.

Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:

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.