Now what is repentance? It is not merely contrition, though contrition is a necessary part of true repentance. Repentance is action, or if you will, counter-action: the turning away from sin toward God in faith. Not that any man will ever set aside the struggle against sin (Rom 7) - that will be with us always until this sinful flesh receives the wages of sin in death. And yet the Scriptures are clear that one cannot continue in willful, deliberate wickedness and claim faith in Christ. That is not faith, but mere knowledge, a knowledge which even the demons have...and shudder. Luther:
On the other hand, if certain sectarists would arise, some of whom are perhaps already extant, and in the time of the insurrection [of the peasants] came to my own view, holding that all those who had once received the Spirit or the forgiveness of sins, or had become believers, even though they should afterwards sin, would still remain in the faith, and such sin would not harm them, and [hence] crying thus: "Do whatever you please; if you believe, it all amounts to nothing; faith blots out all sins," etc.—they say, besides, that if any one sins after he has received faith and the Spirit, he never truly had the Spirit and faith: I have had before me [seen and heard] many such insane men, and I fear that in some such a devil is still remaining [hiding and dwelling].
43] It is, accordingly, necessary to know and to teach that when holy men, still having and feeling original sin, also daily repenting of and striving with it, happen to fall into manifest sins, as David into adultery, murder, and blasphemy, that then faith and the Holy Ghost has departed from them [they cast out faith and the Holy Ghost]. For the Holy Ghost does not permit sin to have dominion, to gain the upper hand so as to be accomplished, but represses and restrains it so that it must not do what it wishes. But if it does what it wishes, the Holy Ghost and faith are [certainly] not present. For St. John says, 1 John 3:9: Whosoever is born of God doth not commit sin, ... and he cannot sin. And yet it is also the truth when the same St. John says, 1:8: If we say that we have no sin, we deceive ourselves and the truth is not in us. (SA III.3.42ff. See also Walther's L&G Thesis X.)
And thus with St. John we know that "there is sin that leads to death; I do not say that one should pray for that. All lawlessness is sin, but there is sin that does not lead to death" (I John 5:16-17). Though we still struggle against sin daily and will not be able to fully overcome in that struggle until death ("still having and feeling original sin, also daily repenting of and striving with it"), we are saved by Christ - and so this sin does not lead to death. Yet there is a sin that leads to death, that drives off faith and the Holy Spirit: sin that is clung to, willfully chosen against better knowledge, etc.
The example that Luther chooses from the Scriptures are instructive. Looking at a woman lustfully is adultery in the heart. It is a rare gift among men to be able to walk down a street full of beautiful women and avoid temptation to this form of adultery. Indeed, most men must struggle against this all their days (and even the pagan Socrates expressed gladness at the coming of old age so that this struggle was somewhat lessened.). Yet Christians struggle against it. They do not embrace the temptation, choose it, and act upon it. When David did that he "cast out faith and the Holy Spirit." He lost his faith. He reverted to pagan status.
I'd say it's likewise with Peter's apostasy. What Christian can claim such an iron clad faith that he never has thoughts of doubt? Who has never wondered in his heart of hearts, "Is all this just so much hogwash?" The temptation to unbelief, to deny Christ, will be with us always in this life in the flesh. Christians struggle against that temptation, even praying as they doubt, "Lord I believe, help Thou mine unbelief!" But when the unbelief is embraced, chosen, acted upon deliberately: "faith and the Holy Spirit have departed." Peter lost his faith. He reverted to pagan status.
As the Apology says, "But since we speak of such faith as is not an idle thought, but of that which liberates from death and produces a new life in hearts, [which is such a new light, life, and force in the heart as to renew our heart, mind, and spirit, makes new men of us and new creatures,] and is the work of the Holy Ghost; this does not coexist with mortal sin [for how can light and darkness coexist?], but as long as it is present, produces good 65] fruits."
[For a much fuller discussion see the Lutheran dogmaticians' discussion of mortal sin under the locus on faith.]
So what is the point of preaching? To change the behavior of men in will, thought, and action. The point of preaching is to turn the will from unbelief to belief, to change thoughts from what is impure to what is pure, to change actions from sins to righteousness. This change is effected only by the power of the Holy Spirit "when and where He pleases" utilizing the Word and Sacraments as means, which are distributed through the Holy Ministry (AC V). So the point of preaching is "repentance and the forgiveness of sins." The latter is only received with the former. The former is not true repentance (but merely a "sorrow of the world that worketh death," I Cor 7:10) unless faith is added to receive the latter.
That might be called "preaching for conversion." I suppose we could also speak of "preaching to prevent reversion:" to warn against the dangers of acting as David and Peter did, to encourage a living and active faith, to strengthen the smoldering wick, etc. But I would still call this preaching "repentance and the forgiveness of sins;" the point is still to encourage faith in the will, purity in thoughts, godliness in actions. And it still all happens by the power of the Spirit operating through the Word and Sacraments. And the Sunday sermon will probably always be a mixture of this preaching for conversion and preaching to prevent reversion.
So, on this Gaudete Sunday, let us preachers of the Word rejoice, going forth in confidence, knowing that the Holy Spirit means to change men by the power of the Word which He has placed in the mouths of us jars of clay.