Catechesis: The Third Use of the Law

For this reason, too, believers require the teaching of the law: so that they do not fall back on their own holiness and piety and under the appearance of God’s Spirit establish their own service to God on the basis of their own choice without God’s Word and command.—FC SD VI:20

However, the Law of God prescribes good works for believers, so that it may at the same time show and indicate, as if in a mirror, that they are still imperfect and impure in this life. For we must say with our dear Paul, “Even if I am not aware of anything against myself, I am not thereby justified.”—FC SD VI:21