Romans 7:7, Galatians 3:24, Romans 3:20
The Law as Mirror
Martin Luther described the law as a mirror — it does not clean the face, but it shows what needs to be cleaned. This is one of the law's primary functions: to expose sin.
Paul's statement is arresting: "If it had not been for the law, I would not have known sin. For I would not have known what it is to covet if the law had not said, 'You shall not covet.' " (Romans 7:7). The law does not create sin — but it reveals it, names it, and makes it visible.
The Three Uses of the Law
The Reformation tradition identified three distinct uses of God's law:
The civil use — the law's function in restraining evil in society. Even where the gospel is not known, God's moral law, written on every human conscience (Romans 2:15), provides a basis for civil order and the restraint of the worst expressions of human sinfulness.
The theological use (the mirror) — the law's function in exposing sin and driving the sinner to Christ. "The law was our guardian until Christ came, in order that we might be justified by faith." (Galatians 3:24). The law is like a schoolmaster: it shows us our sinfulness and drives us to the only One who can deal with it. "Through the law comes knowledge of sin." (Romans 3:20).
The normative use — the law's function as a guide for the redeemed life. For the person who has been justified by faith, the moral law continues to describe the shape of a life that pleases God. Not as a means of earning salvation — that question has been settled — but as a description of what love for God and neighbour looks like in practice.
Law and Gospel
One of the most important distinctions in all of Protestant theology is between law and gospel. The law exposes sin and demands obedience; the gospel announces forgiveness and provides righteousness. The law kills; the gospel makes alive.
Both are necessary. A Christianity of only gospel becomes antinomian — lawless, treating grace as permission to sin (Romans 6:1). A Christianity of only law becomes legalistic — crushing, joyless, self-reliant. Holding law and gospel together — distinguishing them clearly but never separating them — is one of the marks of healthy biblical theology.