In the doctrine of the antinomians there was this statement: “If somebody were an adulterer, provided only that he believed, he would have a gracious God.” But what kind of church will it be, I ask, in which so awful a statement is heard? A distinction is necessary, and it should be taught that adulterers or sinners are of two kinds: some who become aware of their adultery or sin to such an extent that they shudder with their whole heart and begin to repent earnestly, and not only feel sorry for what they have done but also sincerely desire and endeavor never again to commit anything like it. These are not smug in their sin; they are thoroughly frightened, and they dread God’s wrath. If they take hold of the Word of the Gospel and trust in the mercy of God for Christ’s sake, they are saved and have forgiveness of sins through faith in Christ.
Even though the others, whether adulterers or sinners, are unable to excuse their sin, they nevertheless feel no sorrow about it. On the contrary, they are glad that they have achieved their desire. They look for opportunities to commit sins and smugly indulge in them. Because these people do not have the Holy Spirit, they cannot believe; and he who preaches to such people about faith deceives them.
This sickness demands a different medicine, namely, that you say with Paul: “God will judge the adulterous” (Heb. 13:4); “They will not see the kingdom of God” (cf. 1 Cor. 6:9–10); and “Without chastity no one can please God” (Rom. 8:8). Hence they are defiled and under the wrath of God.
Such sledges are needed to crush these rocks. Abraham does not indulge in sins, but long before this he was truly humbled in spirit. Therefore the Lord comforts him, for He takes pleasure in a smoking flax. Therefore He tends it carefully, in order that it may burst into flames.
But the people of Sodom are like crags and very hard rocks. In their case brimstone, lightning from heaven, and thunder are needed. Those foolish and lying prophets who maintain that the Law should not be taught in the church and that, in general, no one should be rebuked too severely or burdened in his conscience are not aware of this.
If this is true, however, why does the Lord want the example of Sodom preserved in His very church and taught by Abraham? Moreover, in view of the fact that He adds “in order that they may fear the Lord,” do not those who want only the promises to be taught exclude the doctrine of the fear of the Lord entirely from the church? Hence the fanatical spirits who confound the entire system of heavenly doctrine in a pernicious manner must be shunned.
But this doctrine of the Law is profitable not only for teaching the fear of the Lord; but, as the Lord adds, it also produces this fruit, that those who are frightened in this way by the judgment and wrath of God practice justice and discernment.
If you divide all Scripture, it contains two topics: promises and threats or benefits and punishments. And, as Bernard states, hearts that are neither softened by kindnesses nor improved by blows are properly called hard. Thus the works of God are also twofold. Works of mercy are those which Paul mentions in Acts 14:17: “He gives rain from heaven, fruitfulness,” etc. He does works of wrath when He also sends a plague, war, and famine in order to frighten and humble the obdurate. Thus in Christ salvation is promised to all who are baptized and believe. On the other hand, judgment and eternal death are threatened to those who do not believe in Christ.
In these circumstances how can or should the preaching of the Law be excluded from the church? Do you not at the same time exclude the fear of God and the majority of the works of God? God certainly does not perform these in order that they may remain hidden, but He wants us to see them and in this way to be led to fear Him. If there were no perils of fire and water, no sudden death and similar evils, I myself would surely not say anything about them and would speak only of God’s kindness and of His benefits. But experience teaches otherwise. Hence to declare that the Law should not be taught in the church is characteristic of men who do not know Christ and are blinded by their pride and wickedness. Previously Moses has set forth many examples of God’s graciousness: when God promised Abraham the Blessed Seed, when He honored him with an outstanding victory, and when He came to him as a guest and ate bread at his home. These events are related by Moses, and nothing else is added to them; but in this passage there is added the command to preach: “He will relate them,” says God, “to his children”; that is: “I want the destruction of Sodom by fire preached in the church.” What is the reason for this? Because the church is never altogether pure; the greater part is always wicked, as the parable of the seed teaches (Matt. 13:3 ff). In fact, the true saints themselves, who are righteous through faith in the Son of God, have the sinful flesh, which must be mortified by constant chastening, as Paul says (1 Cor. 11:31): “If we would judge ourselves, we would not be chastened by the Lord.” Therefore keep this passage in mind. It is adequate by itself to refute the antinomians.
— Bl. Dr. Martin Luther, Lectures on Genesis (18:19), LW 3:224-225