God said, “No, Sarah your wife will have a son, and you will name him Isaac. I will make my agreement with him to be an agreement that continues forever with all his descendants.
Stay in this land, and I will be with you and bless you. I will give you and your descendants all these lands, and I will keep the oath I made to Abraham your father.
When the rainbow appears in the clouds, I will see it and I will remember the agreement that continues forever between me and every living thing on the earth.”
He was too old to have children, and Sarah could not have children. It was by faith that Abraham was made able to become a father, because he trusted God to do what he had promised.