“One errs if he thinks he can preach the Law in such a way that it does not accuse but only guides. It might be possible that it serve as a guide, but it always accuses. Accordingly, the “third use” (or “third function”) of the Law (FC VI) is best explained in this way: the Law is indeed to be preached to Christian congregations in this world because the Christian who is living in the time of the overlapping aeons is simul justus et pecator, “simultaneously saint and sinner.” Because he is peccator–sinner–he still needs to hear the Law, to curb his behaviour and to accuse him of his sin. The notion that, with justification all taken care of and conveniently out of the way, one can forget the old man and preach the Law in a way so as to make it function as the preacher chooses to “use” it, so that there comes about an independent “third use” which guides the new man in the doing of good works, is foreign to Scripture and the Lutheran Confessions. Such a notion involves a hopeless oversimplification that can be held to only if one avoids deeper meditation (or even self-examination) on the nature of the Christian person as simul justus et peccator and on the nature of nomos as a relationship to God in which lex semper accusat. Awareness of answerability eventuates in an awareness of being out of a right relationship to God and of being totally unable to get into a right relationship. Paul says it succinctly (3:20): “through Law is recognition of Sin.” 

– Jonathan Grothe. The Justification of The Ungodly p.g. 164-165