On the Law’s validity in the Church:

     Here the law has its necessary and abiding place in the preaching of the church as well as in the lives of its members. It serves not in the construction of the new man but in the destruction of the old. It must incessantly tell us that we lie when we say we have no sin. It incessantly exercises its “proper function” since it is “always accusing” and since it never can be anything else but that. To be led by the Spirit means not only to be led by Christ, but also to be driven to him, since we are constantly in anguish by reason of the law’s incessant accusations. The “proper function” of the law is, in the language of the old dogmaticians, the usus elenchticus, its function in “exposing” our sin and “convicting” us of sin, and therefore also always the usus paedagogicus which drives us to Christ.
     In order to achieve this goal Christ himself also carries on his “strange work,” his opus alienum, whereby (in the well known words of the Formula of Concord) he “takes the law into his own hands and explains it spiritually; thus he reveals his wrath from heaven over all sinners and shows how great this wrath is. This directs the sinner to the law, and there he really learns to know his sin, an insight that Moses could never have wrung out of him.” Without this spiritual exegesis provided by Christ, the law, says Luther, leads either to the securitas of the Pharisees or to desperatio, to doubting God, to the paralyzing fear of death. But when Christ then moves on to his proper office of speaking the word of forgiveness, the diabolica desperatio is transformed into an evangelica desperatio; man’s doubting of God is transformed into his doubting of himself, and the dire plague of “security” becomes the daring certainty of faith.

-Werner Elert. Law and Gospel