by Martin Luther
And here is the solution of the question which the Diatribe repeats so often all through the book: 'if we can do nothing, what is the purpose of all the laws, precepts, threats and promises?' Paul here gives the answer: 'by the law is the knowledge of sin' (Rom 3:19) His answer to the question is far different than the ideas of man, or of 'free-will". He does not say that 'free-will' is proved by the law, nor that is cooperates unto righteousness; for by the law comes, not righteousness, but knowledge of sin. This is the fruit, the work, the office of the law; it is a light to the ignorant and blind, but one that displays disease, sin, evil, death, hell and the wrath of God. It does not help nor set them free from these things; it is consistent merely to point them out. When a man discovers the sickness of sin, he is cast down and afflicted; nay, he despairs. The law does not help him; much less can he heal himself. Another light is needed to reveal a remedy. This is the voice of the gospel, which displays Christ as the Deliverer from all these evil things. But neither reason, nor 'free-will' points to him; how could reason point to him, when it is itself darkness and needs the light of the law to show it its own sickness, which by its own light it fails to see, and thinks is sound health?
So too in Galatians, dealing with the same point asks: 'Wherefore then serveth the law?' (3.19) He does not answer, as the Diatribe does, that is proves the existence of 'free-will', but says, 'it was added because of transgressions until the seed should come to whom He had made promise.' 'Because of transgressions', he says; not, indeed, to restrain them, as Jerome dreams, for Paul is arguing that the removing and the restraining of sins by the gift of righteousness was promised to the seed that was to come; but to increase transgressions, as he says in Romans 5: 'The law entered, that sin might abound.' (v. 20). But that sins were not committed in abundance without the law; but then were not known to be transgressions and sins of such awful import, and the most and greatest of them were held to be righteousness! As long as sins are unknown. there is no room for a cure, and no hope of one; for sins that think they betoken health and need no physician will not endure the healers' hand. The law is therefore necessary to give knowledge of sin, so that the proud man, who thought he was whole, may be humbled by the discovery of his own great wickedness, and sign and pant after the grace that is set forth in Christ.
See, then, how simple the statement is: 'By the law is the knowledge of sin.' Yet this Text alone has power enough to confound and overthrow "free-will." For if it is truth that 'free-will' of itself does not know what sin and evil are, as the Apostle says here in Romans 7 ('I had not known that covetousness was sin, except the law had said, "Thou shallt not covet" [vv 7, 8], how can it ever know what righteousness and good are? And if it is ignorant of righteousness, how can it endeavor to attain it. We do not know the sin in which we were born, in which we live, and move, and have our being, and which, moreover, lives, moves and reigns in us; how then should we know that righteousness which reigns outside us in heaven? Paul's words make this wretched 'free-will' to be utterly and completely non-existent!
From Luther's Bondage of the Will (free eBook)