For it is written that Abraham had two sons, one by the bondwoman and one by the free woman.
But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise.
This is allegorically speaking, for these [women] are two covenants: one [proceeding] from Mount Sinai bearing children who are to be slaves; she is Hagar.
Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children.
But the Jerusalem above is free; she is our mother.
For it is written, "REJOICE, BARREN WOMAN WHO DOES NOT BEAR; BREAK FORTH AND SHOUT, YOU WHO ARE NOT IN LABOR; FOR MORE NUMEROUS ARE THE CHILDREN OF THE DESOLATE THAN OF THE ONE WHO HAS A HUSBAND."
And you brethren, like Isaac, are children of promise.
But as at that time he who was born according to the flesh persecuted him [who was born] according to the Spirit, so it is now also.
But what does the Scripture say? "CAST OUT THE BONDWOMAN AND HER SON, FOR THE SON OF THE BONDWOMAN SHALL NOT BE AN HEIR WITH THE SON OF THE FREE WOMAN."
So then, brethren, we are not children of a bondwoman, but of the free woman.