Abraham And The Promised Child

Gen 17:15 And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.
Gen 17:16 And I will bless her, and give thee 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.
Gen 17:17 Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?
Gen 17:18 And Abraham said unto God, O that Ishmael might live before thee!
Gen 17:19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.
Gen 17:20 And as for Ishmael, I have heard thee: 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.
Gen 17:21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year.
Gen 17:22 And he left off talking with him, and God went up from Abraham.