But God replied, “No—Sarah, your wife, will give birth to a son for you. You will name him Isaac, and I will confirm my covenant with him and his descendants as an everlasting covenant. As for Ishmael, I will bless him also, just as you have asked. I will make him extremely fruitful and multiply his descendants. He will become the father of twelve princes, and I will make him a great nation. Genesis 17:19-20.
God appeared to Abraham, when he was ninety-nine years old. God changed from Abram’s to Abraham, and again promised a son would be born. Sarai’s name would be changed to Sarah. Both Abraham and Sarah, who was 90 years old, were beyond the age of having children. God, in all His Power and Majesty, kept His promise, and Isaac was born.
Names matter. God’s Covenant matters. Genesis 17:8 – I will fulfill My promise to give you and your descendants the land of Canaan, where you now live as foreigners. I will place all of Canaan into your hands to be yours forever. I will be your God and their God forever. Matthew 28:19-20 contains the forever promise!
Jesus, of the lineage of Abraham, gives this word to His disciples. “Therefore, go and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit. Teach these new disciples to obey all the commands I have given you. And be sure of this: I am with you always, even to the end of the age.”
Being descendants of Abraham doesn’t make them truly Abraham’s children. For the Scriptures say, “Isaac is the son through whom your descendants will be counted,” though Abraham had other children, too. This means that Abraham’s physical descendants are not necessarily children of God. Only the children of the promise are considered to be Abraham’s children. Romans 9:7-8.