The contrast between Abraham's two sons is established in terms of their social status (v. 22) and the manner of their birth (v. 23). Ishmael's mother, Hagar, was Abraham's slave; Isaac's mother, Sarah, was Abraham's wife, a free woman. Since the social status of the mothers determined the social status of their sons, Ishmael was a slave and Isaac was free. Furthermore, there was nothing supernatural about Ishmael's birth: it happened in the ordinary way, as a natural result of the sexual union of Abraham and Hagar. NIV's in the ordinary way is a good paraphrase of "according to the flesh." In this context flesh is not used as a negative, judgmental term; it simply indicates that Ishmael's birth was not caused by anything except the normal biological processes of conception and birth. On the other hand, Isaac was born as the result of a promise. The only way that Abraham's sexual union with his aged, barren wife Sarah could have resulted in conception and birth was by the supernatural fulfillment of the promise of God.
So far Paul has simply summarized the biblical narrative of Abraham's two sons. But what a dramatic contrast his simple summary sets forth: slavery by natural birth and freedom by supernatural birth! It does not take much imagination to see how this contrast could be effectively used to illustrate and apply the truth already given in this letter. If you have only experienced natural birth, you are by nature a slave. But if you have experienced supernatural birth by the fulfillment of God's promise in your life, you are by God's grace set free. Before Paul develops these personal implications, however, he sets up a series of allegorical comparisons.
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