For it is written in the law about Abraham’s two sons [Gen. 16]; one [was] by his slave woman [Hagar] and the other by the free woman, [his wife, Sarah].
Now the son [Ishmael], born to the slave woman, was born simply of their natural relationship; but the son [Isaac], born to the free woman, was born in fulfillment of a [divine] promise.
What does the Scripture say about this matter? [It says, Gen. 21:10-12], “Get rid of the slave woman [Hagar], with her son [Ishmael]; for the son of the slave woman [Ishmael] will not inherit [what God promised to Abraham and his descendants], along with the son of the free woman [i.e., Isaac].”