'Undertaking' in the Bible

Now this I say: The law, which came four hundred and thirty years after, does not put an end to the agreement made before by God, so as to make the undertaking without effect.

What then is the law? It was an addition made because of sin, till the coming of the seed to whom the undertaking had been given; and it was ordered through angels by the hand of a go-between.

However, the holy Writings have put all things under sin, so that that for which God gave the undertaking, based on faith in Jesus Christ, might be given to those who have such faith.

And if you are Christ's, then you are Abraham's seed, and yours is the heritage by the right of God's undertaking given to Abraham.

Now the son by the servant-woman has his birth after the flesh; but the son by the free woman has his birth through the undertaking of God.

Now we, brothers, as Isaac was, are the children of the undertaking of God.