Tell me, you who desire to be under law, do you not hear the law? For it is written that Abraham had two sons, one by a slave and one by a free woman. But the son of the slave was born according to the flesh, the son of the free woman through promise.
Now this is an allegory: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children.
But the Jerusalem above is free, and she is our mother. For it is written, "Rejoice, O barren one who does not bear; break forth and shout, you who are not in travail; for the children of the desolate one are many more than the children of her that is married."
Now we, 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. But what does the scripture say? "Cast out the slave and her son; for the son of the slave shall not inherit with the son of the free woman."
So, brethren, we are not children of the slave but of the free woman.