Sarah, Isaac, and Ishmael — The Covenant Son Named before His Birth
God changes Sarai’s name to Sarah and states with new precision that the promised son will come through her. Abraham laughs at the human impossibility of a hundred-year-old man and a ninety-year-old woman having a child and asks that Ishmael might live before God. God answers both parts: Ishmael will indeed be blessed, fruitful, multiplied, and made into a great nation, but the covenant will be established with Isaac, whom Sarah will bear at the appointed time.
Romans 4 returns directly to Abraham and Sarah’s physical impossibility—the aged body of Abraham and the deadness of Sarah’s womb—to describe faith in the God able to fulfill His promise. Romans 9 later emphasizes the distinction between ordinary physical descent and the specifically promised line: “In Isaac shall thy seed be called,” and the children of promise are distinguished within Abraham’s wider descendants. Galatians 4 likewise uses Isaac and Ishmael in Paul’s inspired allegory about promise and slavery.