After the flesh. Ishmael was the son of human devising, of human effort to bring the covenant promises to fruition. Actually, Ishmael was a living testimony to Abraham’s lack of faith.
By promise. Isaac was the son of promise, the son of faith (Gen. 12:3; 13:14-16; 15:4; 17:3-6, 19-21). The record of his birth is one of remarkable divine interposition (see Gen. 18:10; 21:1, 2; Heb. 11:11, 12). Every circumstance of Isaac’s birth emphasizes faith. The faith of Abraham is an example of Christian faith (see Rom 4:16-25). He believed God’s promises when their fulfillment seemed humanly impossible. Isaac was pre-eminently the son of God’s promise and Abraham’s faith. Paul here places emphasis on the fact that the son born into a state of inferiority and slavery, and the son of a freewoman, into a life of freedom.