God said to Abraham, “Do not let it distress you because of Ishmael and your maid; whatever Sarah tells you, listen to her and do what she asks, for your descendants will be named through Isaac. [Rom 9:7]
Isaac prayed to the Lord for his wife, because she was unable to conceive children; and the Lord granted his prayer and Rebekah his wife conceived [twins].
Now these facts are about to be used [by me] as an allegory [that is, I will illustrate by using them]: for these women can represent two covenants: one [covenant originated] from Mount Sinai [where the Law was given] that bears children [destined] for slavery; she is Hagar.