for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar. For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. But Jerusalem which is above is free, which is the mother of us all. For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not, for the desolate hath many more children than she which hath an husband. Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless, what saith the Scripture? Cast out the bond woman and her son: for the son of the bond woman shall not be heir with the son of the free woman. And we, brethren, are not children of the bond woman, but of the free, in the liberty wherewith Christ hath made us free."  This interpretation of the passage, handed down to us with apostolic authority, shows how we ought to understand the Scriptures of the two covenants -- the old and the new. One portion of the earthly city became an image of the heavenly city, not having a significance of its own, but signifying another city, and therefore serving, or "being in bondage." For it was founded not for its own sake, but to prefigure another city; and this shadow of a city was also itself foreshadowed by another preceding figure. For Sarah's handmaid Agar, and her son, were an image of this image. And as the shadows were to pass away when the full light came, Sarah, the free woman, who prefigured the free city (which again was also prefigured in another way by that shadow of a city Jerusalem), therefore said, "Cast out the bond woman and her son; for the son of the bond woman shall not be heir with my son Isaac," or, as the apostle says, "with the son of the free woman." In the earthly city, then, we find two things -- its own obvious presence, and its symbolic presentation of the heavenly city. Now citizens are begotten to the earthly city by nature vitiated by sin, but to the heavenly city by grace freeing nature from sin; whence the former are called "vessels of wrath," the latter "vessels of mercy."  And this was typified in the two sons of Abraham, -- Ishmael, the son of Agar the handmaid, being born according to the flesh, while Isaac was born of the free woman Sarah, according to the promise. Both, indeed, were of Abraham's seed; but the one was begotten by natural law, the other was given by gracious promise. In the one birth, human action is revealed; in the other, a divine kindness comes to light.