The son of the slave woman was born in the usual way. But the son of the free woman was born because of God's promise.
All who are acceptable because of their faith simply say, “The message is as near as your mouth or your heart.” And this is the same message we preach about faith.
In the Scriptures we learn that Abraham had two sons. The mother of one of them was a slave, while the mother of the other one had always been free.
Even when Sarah was too old to have children, she had faith that God would do what he had promised, and she had a son.