[T]herefore, Paul teaches, at some length and with considerable beauty, that if you consider the true and best use of the Law, it is nothing but some sort of discipline toward righteousness. It humbles men and makes them ready for the righteousness of Christ, if it performs its proper function, that is, if it makes them guilty, terrifies them, makes them conscious of sin, wrath, death, and hell. When this has happened, the presumption of their own righteousness and holiness disappears, and Christ, with His blessings, begins to become sweet. Therefore the Law is not against the promises of God; it is for them. Although it does not fulfill the promise and does not grant righteousness, still, in its use and function, it humbles us and thus makes us ready for the grace and blessing of Christ.
Luther’s Works, volume 26 (331)