Wherefore also He drove him out of Paradise, and removed him far from the tree of life, not because He envied him the tree of life, as some venture to assert, but because He pitied him, and did not desire that he should continue a sinner forever, nor that the sin which surrounded him should be immortal, and evil interminable and irremediable.
But He set a bound to his state of sin, by interposing death, and thus causing sin to cease, putting an end to it by the dissolution of the flesh, which should take place in the earth, so that man, ceasing at length to live to sin, and dying to it, might begin to live to God.
For this end did He put enmity between the serpent and the woman and her seed, they keeping it up mutually:
He, the sole of whose foot should be bitten, having power also to tread upon the enemy’s head; but the other biting, killing, and impeding the steps of man, until the seed did come appointed to tread down his head.
This seed was born of Mary, of whom the prophet speaks: “Thou shalt tread upon the asp and the basilisk; thou shalt trample down the lion and the dragon”—indicating that sin, which was set up and spread out against man, and which rendered him subject to death, should be deprived of its power, along with death, which rules over men;
and that the lion, that is, antichrist, rampant against mankind in the latter days, should be trampled down by Him;
and that He should bind “the dragon, that old serpent” and subject him to the power of man, who had been conquered so that all his might should be trodden down.
Now Adam had been conquered, all life having been taken away from him: wherefore, when the foe was conquered in his turn, Adam received new life; and the last enemy, death, is destroyed, which at the first had taken possession of man.
Therefore, when man has been liberated, “what is written shall come to pass, Death is swallowed up in victory. O death, where is thy sting?”
This could not be said with justice, if that man, over whom death did first obtain dominion, were not set free. For his salvation is death’s destruction. When therefore the Lord vivifies man, that is, Adam, death is at the same time destroyed.
Irenaeus of Lyons (2nd century AD – c. 202): Adversus Haereses, 3,23, 5-7.