Genesis 4:18

That Cain remained alive until the seventh generation is clear. First, … it had been so decreed concerning him. Second, the length of the lives of those first generations also testifies to it. For if his father Adam remained alive until the ninth generation, that of Lamech, and was gathered from the world in the fifty-sixth year of Lamech, it is no great thing that Cain should remain until the seventh generation.

The text runs: "Methushael begot Lamech, who took two wives: the name of the one was Ada, and the name of the other Sella. And Ada brought forth Jobel; who was the father of such as dwell in tents, and of herdsmen. His brother's name was Jubal. He was the father of all those who play the psaltery and cittara. Sella bore Tobel; he was the forger of all instruments of bronze and iron. The sister of Tobel was Noema." This is as far as the line of descent from Cain is carried. There are eight generations in all, including Adam. The seventh is that of Lamech, who was the husband of two wives; the eighth is that of his children, among whom is the woman who is mentioned by name. What is here delicately intimated is that to the very end of its existence the earthly city will be propagated by physical births proceeding from the union of the sexes. This is why we are given the proper names of the wives of the last man mentioned as begetting children—a practice unheard of before the flood, except in the case of Eve.

Moreover, Enoch begot Irad, and Irad begot Mehujael, and Mehujael begot Methushael, and Methushael begot Lamech, who took two wives: the name of one was Adah, and the name of the other was Zillah, and Adah bore Jabal. The progeny of Cain is counted up to the seventh generation, and in this generation, he himself was killed by Lamech, being freed from the sevenfold punishments which he had long borne. In this entire offspring that was born from him, having been also polluted by the adultery of Lamech adding to the curse, perished with the coming flood. Defiled indeed by adultery in the sixth generation, but destroyed by the flood in the seventh generation, in which it is spiritually conveyed that the city of the impious, that is, the entire society of the reprobate, will corrupt itself with crimes in the six ages of this world, but in the seventh, which is in the future, it will perish eternally. For just as Enoch, the seventh from Adam, was taken up into paradise and man did not see him, because he lives in peace with God, so the seventh from Cain, Jabal, is interpreted as "Change," and he was destroyed by the flood with his brothers and all his progeny, and man did not see him, because he was punished with eternal death, and from the glory of the world which he loved, he was changed into the punishment which he did not foresee. In these, it is clearly indicated, as we have said, that the holy tend towards rest in another life after the six ages of this world, and the reprobate towards punishment, as also the history narrated by the Lord about the poor Lazarus and the proud rich man clearly proves.