As a saint, the Christian keeps the law already and does not need to be told to do so. As a sinner, the Christian does not keep the law and needs to be told to do so.
Upon hearing the law, the Christian daily repents and turns to the promise of forgiveness in the means of grace. That is the only way the Christian is truly sanctified and renewed by the Spirit.
For that reason, the new nature has in itself no need of hearing the Decalogue from a pastor or of reading the Decalogue in the Scriptures. The Christian has such a need, still having the old, sinful nature. That is why Decalogue is to be preached to Christians and not only to unbelievers, not that the new nature needs it.
The response of the Christian to the Decalogue is to slay the old nature by returning to baptism in repentance and faith in the gospel since the Decalogue in itself can never sanctify. (While the Decalogue tells us what to do, it does not tell us how to do it.) Sanctification comes only with justification as the sinner-saint daily hears the Decalogue and dies and rises again in baptism to walk in newness of life.
The necessity of properly distinguishing the old nature from the new nature is clear in the writings of Luther. As Article VI of the Formula of Concord says, were it not for the fact that did the Christian has the sinful nature, there would be no need for hearing the Decalogue.
Luther’s stress on the distinction between the old nature and the new nature is striking in how much importance it gives to that teaching, as found in Paul’s letters, compared to the minor role it has in Calvinism and Reformed theology more generally. An error of Reformed theology is to inadequately make the distinction, leading to confusion of law and gospel.*
There should be no doubt that he who is born again cannot sin, that sin is lawlessness, and that anyone who sins has not been born of God according to St. John’s first epistle. At the same time, no good thing dwells in the Christian according to St. Paul. Whereas Calvinists and other Reformed Protestants harmonize the two scriptural teachings into a progressive view of sanctification in accordance with their reason, Lutherans embrace the paradox by recognizing that they speak of the new nature and the old nature, respectively.* In this, they follow the apostolic texts themselves. For example, Romans 7:18 clarifies that no good thing dwells in the flesh of the Christian.
The new nature already knows the law and has no need to hear it preached, but the Christian needs it to reprove him. In the words of the Formula of Concord,
“And, indeed, if the believing and elect children of God were completely renewed in this life by the indwelling Spirit, so that in their nature and all its powers they were entirely free from sin, they would need no law, and hence no one to drive them either, but they would do of themselves, and altogether voluntarily, without any instruction, admonition, urging or driving of the Law, what they are in duty bound to do according to God’s will; just as the sun, the moon, and all the constellations of heaven have their regular course of themselves, unobstructed, without admonition, urging, driving, force, or compulsion, according to the order of God which God once appointed for them, yea, just as the holy angels render an entirely voluntary obedience . . .
“For the old Adam, as an intractable, refractory ass, is still a part of them, which must be coerced to the obedience of Christ, not only by the teaching, admonition, force and threatening of the Law, but also oftentimes by the club of punishments and troubles, until the body of sin is entirely put off, and man is perfectly renewed in the resurrection, when he will need neither the preaching of the Law nor its threatenings and punishments, as also the Gospel any longer; for these belong to this [mortal and] imperfect life.”
Article V of the Apology of the Augsburg Confession also explains putting off the body of sin and rising to new life in terms of repentance and faith:
“. . . there is first terror and anxiety in the conscience. Thus contrition and faith go side by side. One is putting off the body of sins; the other is the rising again through faith. Neither ought these words, mortification, quickening, putting off the body of sins, rising again, to be understood in a Platonic way, concerning a feigned change; but mortification signifies true terrors, such as those of the dying, which nature could not sustain unless it were supported by faith. So he names that as the putting off of the body of sins which we ordinarily call contrition, because in these griefs the natural concupiscence is purged away. And quickening ought not to be understood as a Platonic fancy, but as consolation which truly sustains life that is escaping in contrition. Here, therefore, are two parts: contrition and faith. For as conscience cannot be pacified except by faith, therefore faith alone quickens, according to the declaration, Hab. 2:4; Rom. 1:17: The just shall live by faith.”
Thereby raised, the new nature delights in the law, meditating on God’s word day and night (Psalm 1; 119).
* On the remarks about the Calvinistic branch of Reformed theology, see David Scaer’s article (note 20 of page 243); the top of page 238 is relevant to the continual complaint that Lutherans would be more sanctified if their pastors better understood the third use of the law. David Scaer exhibits a penetrating grasp of Reformed theology. He calls the Lutheran position by contrast “Nestorian” in its teaching on the old Adam and the new Adam striving against each other in the same Christian.