Ambrose of Milan : Indeed, it was not without reason that the mark was set upon Cain, that no one might kill him. Thus it was indicated that evil is not destroyed or removed from the earth. Cain was afraid that he might be killed, because he did not know how to flee. For evil is augmented and amassed by the practice of evil, and it exists without moderation or limit, fights through guile and deceit and is revealed by its deeds and by the blood of the slain, even as Cain also was revealed. — FLIGHT FROM THE WORLD 7.39 Ambrose of Milan : Therefore as a slave he received a sign, but even thus he could not escape death. Thus the sinner is a slave to fear, to cupidity, to avarice, to lust, to malice, to anger, nay, he is a greater slave than if he were set under tyrants. — Letter 37, To Simplician Bede : By no means, he said, shall it be so, but whoever kills Cain shall be avenged sevenfold. You shall not die as you think, and you shall not receive death as a remedy, but you shall live for a long time as an example to others, so that they may not dare to sin in the same way; and it is so far from my wish that you be killed by anyone, that if anyone kills you, he shall be punished sevenfold, that is, with the gravest vengeance, for he will not have refrained from shedding blood despite being warned by the severity of your condemnation. For man should not have killed the man to whom God had given life despite his sin, either for the accumulation of his punishment or for the exemplar of caution and correction of others. For often Scripture signifies fullness by the number seven, as in Leviticus: If you walk contrary to me, and will not listen to me, I will add seven times more plagues upon you (Lev. 26:21). And a little later: And I will strike you seven times for your sins (Lev. 26:24), meaning manifoldly. And in a favorable sense the Psalmist says: Seven times a day I praise you (Ps. 118:164), which is to say in other words: His praise shall constantly be in my mouth (Ps. 34:1). As the ancient interpreters have said: Whoever kills Cain shall pay sevenfold vengeance, this holds the meaning: You shall live up to the seventh generation, and be tormented by the fire of your conscience; so that whoever kills you, according to double understanding, either in the seventh generation or to free you from great torment: not that he who kills Cain should be subjected to sevenfold vengeance, but that he should pay the sevenfold vengeance which have extended through the whole time of Cain, by killing him who was left alive for punishment. For the seventh from Adam was Lamech, who is said to have killed Cain: and this same Lamech fulfilled the sevenfold vengeance which Cain, having sinned sevenfold, deserved, by his death. For his first sin was that, offering to God, he did not divide rightly; the second, that he envied his brother; the third, that he acted deceitfully, saying: Let us go out to the field; the fourth, that he killed him; the fifth, that he brazenly denied: I do not know; the sixth, that he condemned himself, saying: My iniquity is greater than that I can deserve pardon; the seventh, that he did not repent even after being condemned. — Commentary on Genesis (Hexaemeron) Bede : And God set a sign upon Cain, lest anyone who found him would kill him. The very sign by which it was evident that he would always live as a trembling, groaning, wandering, and fugitive; reminded by his own misery, because he could not be killed by just anyone; but whoever would kill him would either deliver Cain himself from great miseries or, by doing this, would subject himself to a sevenfold vengeance. — Commentary on Genesis (Hexaemeron) Bede : And God set a mark upon Cain, so that no one who found him would kill him. The Jewish people, whether under pagan or Christian rulers, have the mark of their law, by which they are distinguished from other nations and peoples, and every emperor or king who finds them in his kingdom finds them with that mark, nor does he kill them, that is, he does not make them cease to be Jews, being distinguished by a certain definite mark of their own observance from the communion of other nations, unless any of them have turned to Christ. — Commentary on Genesis (Hexaemeron) Cosmas Indicopleustes : when Cain petitioned for death through horror at his fratricide, did God not give it, but rather delivered him over to a still heavier punishment, namely, to remain on the earth lamenting and trembling—an evil from which he said death would be a deliverance? God therefore said: Whosoever slayeth Cain, vengeance shall be taken on him sevenfold. — The Christian Topography, Book 5 Jerome : (Verse 15.) Everyone who kills Cain will suffer sevenfold vengeance. Aquila interpreted it sevenfold. Symmachus, the seventh. Theodotion, by the week. On this chapter, our letter to Bishop Damasus exists. — Hebrew Questions on Genesis