For it is written,
Rejoice, you barren that bear not; break forth and cry, you that travail not: for the desolate has many more children than she who has a husband.
All Commentaries on Galatians 4:27 Go To Galatians 4
Augustine of Hippo
AD 430
Sarah signifies the heavenly Jerusalem, having been deserted for a long time by her husband’s embraces because of her perceived sterility. For men such as Abraham did not use women to satisfy lust but for the procreation of offspring. Now to her sterility age had also been added…. The age of Isaac’s parents serves to signify that, new though the people of the New Covenant are, their predestination lies nonetheless with God and is that heavenly Jerusalem of old.