Now all this is an allegory; these [two women] represent two covenants. One covenant originated from Mount Sinai [where the Law was given] and bears [children destined] for slavery; this is Hagar.
And Isaac prayed much to the Lord for his wife because she was unable to bear children; and the Lord granted his prayer, and Rebekah his wife became pregnant.
God said to Abraham, Do not let it seem grievous and evil to you because of the youth and your bondwoman; in all that Sarah has said to you, do what she asks, for in Isaac shall your posterity be called. [Rom. 9:7.]