God said, “Listen to me. I promise that you and Sarah will have a son, and you will call him Isaac. I will confirm my everlasting covenant of love with him and his seed.
Physical descent from Abraham doesn’t guarantee the inheritance, because God has said: “Through Isaac your descendants will be counted as part of your lineage.”
I took your father Abraham from that land and led him through all the land of Canaan. I gave him a son, Isaac, and through Isaac I multiplied his descendants.
“Then God entered into covenant with Abraham, which included the requirement of circumcision. So when he became the father of Isaac, he circumcised him eight days after his birth.
God said, “Please take your son, your only son, Isaac, whom I know you dearly love, and go to the land of Moriah. Offer him up to me as a burnt offering on one of the mountains which I will show you.”
God spoke to Abraham, “Don’t be distressed over the slave woman and her son. Listen to every word Sarah says to you, and do it, for it will be through Isaac your promise of descendants will be fulfilled.