When the sophists speak of original sin, they are speaking only of wretched and hideous lust or concupiscence. But original sin really means that human nature has completely fallen; that the intellect has become darkened, so that we no longer know God and His will and no longer perceive the works of God; furthermore, that the will is extraordinarily depraved, so that we do not trust the mercy of God and do not fear God but are unconcerned, disregard the Word and will of God and follow the desire and the impulses of the flesh; likewise, that our conscience is no longer quiet but, when it thinks of God's judgment, despairs and adopts illicit defenses and remedies. These sins have taken such deep root in our being that in this life they cannot be entirely eradicated, and yet the wretched sophists do not mention them even with a word. Thus, as it always is with correlatives, original sin shows what original righteousness is, and vice versa: original sin is the loss of original righteousness, or the deprivation of it, just as blindness is the deprivation of sight.
This involves much more than the monks think when they restrict original righteousness almost exclusively to chastity. But the soul ought to be given consideration first; thereafter also the body, which has been made so hideous by lust. But in the case of the soul the outstanding fact is this: that the knowledge of God has been lost; that we do not everywhere and always give thanks to Him; that we do not delight in His works and deeds; that we do not trust Him; that when He inflicts deserved punishments we begin to hate God and to blaspheme Him; that when we must deal with our neighbor, we yield to our desires and are robbers, thieves, adulterers, murderers, cruel, inhuman, merciless, etc. The passion of lust is indeed some part of original sin. But greater are the defects of the soul: unbelief, ignorance of God, despair, hate, blasphemy. Of these spiritual disasters Adam, in the state of innocence, had no knowledge.
--Martin Luther, Lectures on Genesis, Luther's Works volume 1:114.