At length satisfaction follows sin, not that papal satisfaction, but the vengeance of God. God indeed pardons and remits sins, but let the sinner not fall asleep; let him not become smug and boast as though of something well done. For God comes with a rod of iron and visits the iniquity of the fathers upon the children to the third and fourth generation (Ex. 20:5). Why? Not because He is satisfied for sin through that punishment, for He is not satisfied by punishments. Nor does He have regard for our satisfactions, but He punishes so that the sinner may not smugly snore away or glory in his evil way but so that sin may bite us, mortify, and compel us to acknowledge its magnitude, to weep, to sigh, and to implore grace and the gracious pardon promised to those who weep and mourn and who repent of their sins and acknowledge them.
But if God did not punish, not only would we overlook sin and snore, but we would daily accumulate other far greater sins. “Sin is couching at the door,” Cain is told (Gen. 4:7), but at length it will unexpectedly awake and crucify you, yet not for your perdition and damnation but for your repentance and change of heart, so that you may acknowledge your sin, groan, cry out, and invoke the mercy of God. They will have paid well for their sin! The Israelites, to be sure, paid quite a heavy penalty for their sins, as is set forth in the story of Exodus, chapter 1. For just as they killed their brother, father, and mothers, so in their turn they suffered infinite evils in Egypt under the tyranny of Pharaoh. Pharaoh really takes their measure in Egypt so that they may remember what a great sin they have committed and not regard sin as a light matter.
If we do not carefully and correctly estimate the magnitude of sin, God Himself will do so; if we do not judge ourselves, the Lord will judge us. But it is a great blessing that He judges and punishes for salvation and not for damnation, as Judas and Saul were judged. But if you idly think and say: “Even though I have offended my parents, what of it?” Then sin is of course asleep, but it will awaken and drive you to say: “How wretched I am! What have I done? Why have I despised God in my father, mother, and teacher? Why have I not obeyed their admonitions?” This will eventually take place; it cannot be otherwise!
Luther’s works, vol. 6: Lectures on Genesis: Chapters 31-37 p 368-369