And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be a hundred and twenty years.
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Augustine of Hippo
By the justice of God the whole human race was delivered into the power of the devil, the sin of the first man passing originally into all persons of both sexes, who were born through conjugal union, the debt of our first parents binding all their posterity. This delivering was first indicated in Genesis, where, when it was said to the serpent, “Earth shall you eat,” it was said to the man, “Earth you are, and into earth shall you return.” The death of the body was foretold by “into earth shall you return,” because he would not have experienced it if he had remained upright as he had been created. But what he says to the living man, “earth you are,” shows that the whole man has been changed into something worse, for “earth you are” is just the same as saying “My spirit shall not remain in those men, because they are flesh.” Hence God showed that he had then delivered man to the devil, to whom he had said, “Earth shall you eat.” .
This generation will not live nine hundred years like the previous generations, for it is flesh and its days are filled with the deeds of flesh. Therefore, their days will be one hundred and twenty years. If they repent during this time, they will be saved from the wrath that is about to come upon them. But if they do not repent, by their deeds they will call down wrath upon themselves. Grace granted one hundred and twenty years for repentance to a generation that, according to justice, was not worthy of repentance. .
His days shall be The meaning is, that man's days, which before the flood were usually 900 years, should now be reduced to 120 years. Or rather, that God would allow men this term of 120 years, for their repentance and conversion, before he would send the deluge. (Challoner)
He spoke therefore to Noe in his 480th year. (St. Augustine) Those who suppose, that he foretold this event 20 years later, think with St. Jerome, that God retrenched 20 years from the time first assigned for penance. The Spirit of the sovereign Judge was fired with contending; or, as others translate it, with remaining quiet as in a scabbard, and bearing with the repeated crimes of men. He resolved to punish them severely in this world, that he might show mercy to some of them hereafter. (St. Jerome, 9. Heb.) (Calmet)
If we suppose, that God here threatens to reduce the space of man's life to 120 years, we must say, at least, that he did it by degrees: for many lived several hundred years, even after the deluge....
Furthermore, lest [God] may seem to be cruel in that he had not given to sinners a place for repentance, he added, “But their days will be years,” that is, they will have years to do repentance. It is not therefore that human life was contracted to years, as many wrongly assert, but that years were given to that generation for repentance, since indeed we find that after the flood Abraham lived years and others more than and years. Since indeed they despised to do repentance, God was unwilling for his decree to await its time, but cutting off the space of twenty years he brought on the flood in the one hundredth year that had been destined for doing repentance.