51 Now, having defended the character of the patriarch, and refuted an accusation arising from these detestable errors, let us avail ourselves of the opportunity of searching out the symbolic meaning, and let us knock with the reverence of faith, that the Lord may open to us the typical significance of the four wives of Jacob, of whom two were free, and two slaves. We see that, in the wife and bond-slaves of Abraham, the apostle understands the two Testaments. But there, one represents each; here, the application does not suit so well, as there are two and two. There, also, the son of the bond-slave is disinherited; but here the sons of the slaves receive the land of promise along with the sons of the free women: so that this type must have a different meaning.
Source: Reply to Faustus the Manichaean (New Advent)