Thematic Bible: The real heir born

And the Lord came to Sarah as he had said and did to her as he had undertaken. And Sarah became with child, and gave Abraham a son when he was old, at the time named by God. And Abraham gave to his son, to whom Sarah had given birth, the name Isaac. read more.
And when his son Isaac was eight days old, Abraham made him undergo circumcision, as God had said to him. Now Abraham was a hundred years old when the birth of Isaac took place. And Sarah said, God has given me cause for laughing, and everyone who has news of it will be laughing with me. And she said, Who would have said to Abraham that Sarah would have a child at her breast? for see, I have given him a son now when he is old.