John Calvin Commentary Genesis 4:15

"And Jehovah said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And Jehovah appointed a sign for Cain, lest any finding him should smite him." — Genesis 4:15 (ASV)

Therefore, whosoever slayeth Cain. Those who think that it was Cain’s wish to perish immediately by one death, so that he would not be agitated by continual dangers, and that the extension of his life was granted to him only as a punishment, have no reason, as far as I can see, for speaking this way.

But far more absurd is the way in which many of the Jews mutilate this sentence. First, they imagine, in this clause, the use of the figure ἀποσιώπησις, according to which something not expressed is understood; then they begin a new sentence, He shall be punished sevenfold, which they refer to Cain.

Still, however, they do not agree about the meaning. Some offer trivial interpretations concerning Lamech, as we will soon declare. Others interpret the passage as referring to the deluge, which happened in the seventh generation. But that is frivolous, since the deluge was not a private punishment for only one family, but a common punishment of the human race.

But this sentence ought to be read continuously, like this: Whosoever killeth Cain, shall on this account, be punished sevenfold. And the causal particle לכן (lekon,) indicates that God would take care to prevent anyone from easily attacking and destroying him; not because God would establish a privilege for the murderer, or would listen to his prayers, but because He would consider posterity, in order to preserve human life.

The order of nature had been terribly violated. What might be expected to happen in the future, when human wickedness and audacity were to increase, unless the fury of others had been restrained by a violent hand? For we know what pestilent and deadly poison Satan presents to us in evil examples, if a remedy is not quickly applied.

Therefore, the Lord declares, if anyone imitates Cain, not only will they have no excuse in his example, but they will be more grievously tormented, because they ought, from Cain's example, to perceive how detestable their wickedness is in the sight of God. For this reason, those who suppose that the anger of God is lessened when people can plead custom as an excuse for sinning are greatly deceived; whereas it is for that very reason all the more inflamed.

And the Lord set a mark. I have recently said that nothing was granted to Cain to favor him, but rather to oppose future cruelty and unjust violence. And so, Moses now says that a mark was set upon Cain that would strike terror into all, because they might see, as in a mirror, the tremendous judgment of God against bloody men. Since Scripture does not describe what kind of mark it was, commentators have conjectured that his body became tremulous. It is enough for us that there was some visible token that would repress in spectators the desire and audacity to inflict injury.