Here’s a few paragraphs from Luther’s introduction to Psalm 51.
This knowledge of sin, moreover, is not some sort of speculation or an idea which the mind thinks up for itself. It is a true feeling, a true experience, and a very serious struggle of the heart, as he testifies when he says (v. 3), “I know (that is, I feel or experience) my transgressions.” This is what the Hebrew word [יָדַע] really means. It does not mean, as the pope taught, to call to mind what one has done and what one has failed to do; but it means to feel and to experience the intolerable burden of the wrath of God. The knowledge of sin is itself the feeling of sin, and the sinful man is the one who is oppressed by his conscience and tossed to and fro, not knowing where to turn. Therefore we are not dealing here with the philosophical knowledge of man, which defines man as a rational animal and so forth. Such things are for science to discuss, not for theology. So a lawyer speaks of man as an owner and master of property, and a physician speaks of man as healthy or sick. But a theologian discusses man as a sinner. In theology, this is the essence of man. The theologian is concerned that man become aware of this nature of his, corrupted by sins. When this happens, despair follows, casting him into hell. In the face of the righteous God, what shall a man do who knows that his whole nature has been crushed by sin and that there is nothing left on which he can rely, but that his righteousness has been reduced to exactly nothing? When the mind has felt this much, the other part of this knowledge should follow. This is not a matter of speculation either, but completely of practice and feeling. A man hears and learns what grace and justification are, what God’s plan is for the man who has fallen into hell, namely, that He has decided to restore man through Christ. Here the dejected mind cheers up, and on the basis of this teaching of grace it joyfully declares: “Though I am a sinner in myself, I am not a sinner in Christ, who has been made Righteousness for us (1 Cor. 1:30). I am righteous and justified through Christ, the Righteous and the Justifier, who is and is called the Justifier because He belongs to sinners and was sent for sinners.”
This is the twofold theological knowledge which David teaches in this psalm, so that the content of the psalm is the theological knowledge of man and also the theological knowledge of God. Let no one, therefore, ponder the Divine Majesty, what God has done and how mighty He is; or think of man as the master of his property, the way the lawyer does; or of his health, the way the physician does. But let him think of man as sinner. The proper subject of theology is man guilty of sin and condemned, and God the Justifier and Savior of man the sinner. Whatever is asked or discussed in theology outside this subject, is error and poison. All Scripture points to this, that God commends His kindness to us and in His Son restores to righteousness and life the nature that has fallen into sin and condemnation. The issue here is not this physical life—what we should eat, what work we should undertake, how we should rule our family, how we should till the soil. All these things were created before man in Paradise and were put into man’s hands when God said (Gen. 1:28), “Have dominion over the fish of the sea and over the birds of the air.” The issue here is the future and eternal life; the God who justifies, repairs, and makes alive; and man, who fell from righteousness and life into sin and eternal death. Whoever follows this aim in reading the Holy Scriptures will read holy things fruitfully.
Luther’s Works, 12:320-321
