Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 3

Chapter 3.—That Sarah’s Barrenness was Made Productive by God’s Grace.

Sarah, in fact, was barren; and, despairing of offspring, and being resolved that she would have at least through her handmaid that blessing she saw she could not in her own person p. 286 procure, she gave her handmaid to her husband, to whom she herself had been unable to bear children.  From him she required this conjugal duty, exercising her own right in another’s womb.  And thus Ishmael was born according to the common law of human generation, by sexual intercourse.  Therefore it is said that he was born “according to the flesh,”—not because such births are not the gifts of God, nor His handiwork, whose creative wisdom “reaches,” as it is written, “from one end to another mightily, and sweetly doth she order all things,” 773 but because, in a case in which the gift of God, which was not due to men and was the gratuitous largess of grace, was to be conspicuous, it was requisite that a son be given in a way which no effort of nature could compass.  Nature denies children to persons of the age which Abraham and Sarah had now reached; besides that, in Sarah’s case, she was barren even in her prime.  This nature, so constituted that offspring could not be looked for, symbolized the nature of the human race vitiated by sin and by just consequence condemned, which deserves no future felicity.  Fitly, therefore, does Isaac, the child of promise, typify the children of grace, the citizens of the free city, who dwell together in everlasting peace, in which self-love and self-will have no place, but a ministering love that rejoices in the common joy of all, of many hearts makes one, that is to say, secures a perfect concord.

Footnotes

Wisd. 8.1.