Genesis 17:15-23; 18:1-17

15 ¶ And God said to Abraham, As for Sarai your wife, you shall not call her name Sarai, but Sarah shall her name be. The word Sarah means Princess. The word Sarai means Contentious. She had an attitude change as well as a name change.
16 And I will bless her, and give you a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her. God goes over this promise to Abraham many times, about his lineage being great. The repetition means it is very important.
17 Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born to him that is an hundred years old? and shall Sarah, that is ninety years old, bear? Abraham laughs because he can hardly believe what God is telling him, at his age of 100 years old, and Sarah is 99 years old. Abraham thinks they are beyond childbearing age. Abraham is astonished at God's news.
18 And Abraham said to God, O that Ishmael might live before you! Abraham already has Ishmael who is now 13 years old, and Abraham loves him. He asks God, why not consider Ishmael?
19 And God said, Sarah your wife shall bear you a son indeed; and you shall call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. Sarah thought it was hilarious that she should have a child at 99 years of age. God differentiates between the child Isaac that's coming and the child that is already in the tent, Ishmael.
20 And as for Ishmael, I have heard you: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation.....