According to Lutheran theology, the believer needs the law only because he is still a sinner, that is to say, he is not yet a perfect believer in every respect, not yet a person who has the absolute principle as the all-determining dynamic of his life. For the Lutheran, therefore, the activity called forth by the law is directed principally to the work itself, to practicing the personal and indeed negative virtues, to subduing evil desires (concupiscentia). The believer’s chief concern is to rid himself more and more of the remnants of his sinful Old Man. Precisely these remnants are recognized in the mirror of the law and are constantly struck by its reprimands. As a believer, however, who no longer is under the law, he has only to conserve the faith he has by always establishing himself in it anew. Here, too, of course, belongs the manifestation of love in his outward activities, if indeed he is to avoid losing the fellowship with God he already has.
According to the Reformed view, on the other hand, a believer becomes secure as far as he himself and his final, definite overpowering of sin are concerned only by doing good works. His chief activity to which the divine law summons him is therefore directed toward outward work, toward the positive shaping of the world according to the divine norm.
This basic difference, which is, to be sure, in itself rather subtle but nevertheless very significant and characteristic with respect to the authority of the law, not only gives Reformed piety its particular quality and spirit, but also has consequences and similarities in other doctrines.
Matthias (Max) Schneckenburger, “Vergleichende Darstellung des lutherischen und reformierten Lehrbegriffs” (A Comparative Presentation of the Lutheran and Reformed Concept of Doctrine), quoted in “The Difference Between The Reformed And The Lutheran Interpretation Of The So-Called Third Use Of The Law” by August Pieper. All italicized print appears in Pieper’s essay.