Then God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. And I will bless her and also give you a son by her; then I will bless her, and she shall be a mother of nations; kings of peoples shall be from her.”
Then Abraham fell on his face and laughed, and said in his heart, “Shall a child be born to a man who is one hundred years old? And shall Sarah, who is ninety years old, bear a child?” And Abraham said to God, “Oh, that Ishmael might live before You!”
Then God said: “No, Sarah your wife shall bear you a son, and you shall call his name Isaac; I will establish My covenant with him for an everlasting covenant, and with his descendants after him. And as for Ishmael, I have heard you. Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly. He shall beget twelve princes, and I will make him a great nation. But My covenant I will establish with Isaac, whom Sarah shall bear to you at this set time next year.” Then He finished talking with him, and God went up from Abraham. (Genesis 17:15-22)
Sarah did not believe it when she overheard God tell her husband that within a year she’d be pregnant; so much did she disbelieve, that she laughed out loud. But she was not alone in her laughter. Abraham laughed as well. And they had good reason to laugh. They were both past the time when people could have children. What God suggested to them was simply not a possibility.
Their disbelieving laughter would be transformed, in less than a year, to the laughter of joy. They called their son “Laughter,” the meaning of the name “Isaac,” because we commonly laugh at the unexpected and the incongruous, and what could be more unexpected and incongruous than a ninety year old woman giving birth to a healthy baby boy?
God promised that the contract that God had made with Abraham would be extended to Isaac, not to Ishmael or anyone else. The line of promise was being narrowed. Ultimately it would focus on the Jewish people exclusively, who would have a special relationship with God and from whom the Messiah was destined to come.
God acted despite their reasonable doubts, despite their lack of faith. In fact, he did not even wait for Abraham to believe his words. He just told Abraham how it would be.