Because it is in the Writings, that Abraham had two sons, one by the servant-woman, and one by the free woman.
And Hagar gave birth to a child, the son of Abram, to whom Abram gave the name of Ishmael.
So she said to Abraham, Send away that woman and her son: for the son of that woman is not to have a part in the heritage with my son Isaac.
Now the son by the servant-woman has his birth after the flesh; but the son by the free woman has his birth through the undertaking of God.
What then do the Writings say? Send away the servant-woman and her son; for the son of the servant-woman will not have a part in the heritage with the son of the free woman.
So, brothers, we are not children of the servant-woman, but of the free woman.