We have an altogether similar example in the pope’s church and in ours; for we have the same Baptism, the same Sacrament of the Altar, the same Keys, and the same Scripture and Word. Finally we have our origin from the same apostles and the same church, as though from one mother Rebecca. Why, then, do we disagree? We apprehend the Word in the Sacrament, follow the call, and discuss the Word in accordance with faith; but they do so in accordance with what is seen on the outside. We declare that when making use of the sacraments one must pay attention to the Word and accept it in faith; they make it a mere work that is performed. Therefore the true church is the one that holds fast to the Word and faith and does not rely on works but hears and follows God when He calls. But the call is the Word through which our nature, which has been corrupted by the devil and sin, is regenerated.
This is not an unimportant disagreement and struggle. And it is not without cause that our adversaries are angry with us in such a hostile manner, for we are debating about the most important matters in the entire world—not about the empire and not about wealth, prestige, or power but about eternal damnation and eternal life. This is the reason why the hatreds are so bitter. And nowhere are they bitterer than in disagreements in the matter of religion, for it is a most important and most serious cause. Are we children of God or children of the devil? They do not want to be children of the devil, but they usurp the glory and the name of true church and therefore condemn and kill us. On the other hand, we condemn them on the basis of the Word, and because of true evidences we are sure of our own salvation. We keep a firm hold on the difference which God established between Abraham and the Babylonians, between Ishmael and Isaac, and between Jacob and Esau, namely, the call by which Abraham was told (Gen. 12:1): “Go from, etc.”; “Through Isaac shall your descendants be named” (Gen. 21:12); and (Gen. 25:23): “The elder shall serve the younger.”
Luther’s works, vol. 4: Lectures on Genesis: Chapters 21-25 p 349-350