Tell me, you who are so eager to have the law for your master, have you never read the law? You will find it written there, that Abraham had two sons; one had a slave for his mother, and one a free woman. The child of the slave was born in the course of nature; the free woman’s, by the power of God’s promise. All that is an allegory; the two women stand for the two dispensations. Agar stands for the old dispensation, which brings up its children to bondage, the dispensation which comes to us from mount Sinai. Mount Sinai, in Arabia, has the same meaning in the allegory as Jerusalem, the Jerusalem which exists here and now; an enslaved city, whose children are slaves. Whereas our mother is the heavenly Jerusalem, a city of freedom. So it is that we read, Rejoice, thou barren woman that hast never borne child, break out into song and cry aloud, thou that hast never known travail; the deserted one has more children than she whose husband is with her. It is we, brethren, that are children of the promise, as Isaac was. Now, as then, the son who was born in the course of nature persecutes the son whose birth is a spiritual birth. But what does our passage in scripture say? Rid thyself of the slave and her son; it cannot be that the son of a slave should divide the inheritance with the son of a free woman.
— Epistle to the Galatians, iv. 21-30.