Luther’s understanding of the law developed from the way Paul speaks of it in contrast with the gospel, particularly in Romans and Galatians. In this context, Paul does not so much describe the law by what it is is, but by what it does. The law accuses, binds, condemns, curses, terrifies, and kills. The gospel acquits, frees, saves, blesses, comforts and brings to life. In this dialectic, Luther saw the law and the gospel as necessary opposites, but with the law always hand-maiden to the gospel - never the other way around.
In the Lutheran confessions, Melanchthon described the accusatory function of the law with the latin phrase lex semper accusat (the law always accuses) and Lutheran theologians have thence referred to this function as the “semper”. So now we have the simul and the semper. Both are necessary to understand Luther - and all the weeping and gnashing of teeth over the third use of the law.
The semper fits properly with the first and second uses of the law, both of which confront me as a sinner, whether Christian or not. First, it always (semper) demands and threatens, thus bringing about some level of restraint, civic righteousness and moral behavior. This promotes a peaceful society, but cannot reconcile me to God. So the law continues to always (semper) accuse and condemn me. It does not remove my guilt, but rather magnifies it - making me always (semper) more aware of my sin, shame and broken relationship with God. And so comes the second use of the law - to dramatize my need for help beyond the law, namely the Savior who fulfilled the law for me and takes away my guilt (the gospel).
Now, however, when I come to the third use, the semper does not fit. In Christ, the new me is fully clothed in the righteousness of Christ. I am no longer subject to the coercion or accusation of the law. So is the semper no longer semper? Has the law changed? No, it continues to accuse - but it accuses Christ. So Christ became guilty, was condemned and crucified. And thus Christ became the end of the law for me.
In view of the cross, I live as the new me and the semper does not apply. Nothing can accuse me or shame me or condemn me. In this new me, I can (by faith) come to the law and delight in it. It is no longer a threat. However, I am not convinced that this is necessarily the point of the new me. In the righteousness of Christ, a return to the Ten Commandments (minus the semper) seems strangely anticlimactic. I think there’s more to it.
What of the possibility that the moral law was indeed fulfilled in Christ and now the “law of Christ”, the “walking in the spirit”, the “fruits of the spirit” - all this is something entirely new - something way different from perfect obedience to moral law - or law of any kind?
The first two uses of the law no doubt continue in effect for the old me. But (to the new me) the third use smells a bit like an old wineskin.