Therefore the strongest argument that can be opposed to the previous one is to be found in the fact that God made a distinction between Isaac and Ishmael and between Jacob and Esau, who were born in the same way of Abraham and Isaac. Nevertheless, Ishmael, no matter how much he desires it, cannot be the heir. No, the seed of the promise, which has the call and, over and above the first birth, has the second and regeneration, is given the preference. But this is the source of perpetual war from the beginning of the world to the end, not about trivialities but about that glorious title “church,” the people of God, the kingdom of heaven, and eternal life. Thus today we are at variance with the church of the pope, which wants to be the people of God and to have possession of the kingdom and the priesthood. They boast that they alone are the church of God which acknowledges God as the Father and worships Him properly. They condemn and persecute us as heretics and the church of the devil. This is what it means that the infants are at variance before they were born; for from the beginning there is a twofold church in the world, just as the seed is twofold: the seed of the serpent and the seed of the woman, which contend and are at variance with each other because of the title “church.” Paul certainly handed down an exceedingly clear and powerful dialectic when he pointed out the difference between the birth and the call. Where there is the birth alone, there is condemnation; for, as John says, “That which is born of the flesh is flesh” (John 3:6) and “not of blood, etc.” (John 1:13). Paul says something else. “Because of His call she was told,” he writes (Rom. 9:11-12); that is, the Word of God and the promise are necessary. Over and above the creature, he who wants to rule and be a son of God must hear Him, not as the God who creates but as the God who calls.
Luther, Martin (1966-01-01). Luther’s Works, Vol. 4: Genesis Chapters 21–25 (Kindle Locations 6816-6830). Concordia Publishing House. Kindle Edition.