Apparently Christians in the second century debated whether Adam had been saved by the death and resurrection of Christ. Read through this passage:
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 for ever, 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, — which 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 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.
All therefore speak falsely who disallow his (Adam’s) salvation, shutting themselves out from life for ever, in that they do not believe that the sheep which had perished has been found. For if it has not been found, the whole human race is still held in a state of perdition. False, therefore, is that man who first started this idea, or rather, this ignorance and blindness — Tatian. As I have already indicated, this man entangled himself with all the heretics. This dogma, however, has been invented by himself, in order that, by introducing something new, independently of the rest, and by speaking vanity he might acquire for himself hearers void of faith, affecting to be esteemed a teacher, and endeavoring from time to time to employ sayings of this kind often [made use of] by Paul: “In Adam we all die;” ignorant, however, that “where sin abounded, grace did much more abound.” Since this, then, has been clearly shown, let all his disciples be put to shame, and let them wrangle about Adam, as if some great gain were to accrue to them if he be not saved; when they profit nothing more [by that], even as the serpent also did not profit when persuading man [to sin], except to this effect, that he proved him a transgressor, obtaining man as the first- fruits of his own apostasy. But he did not know God’s power. Thus also do those who disallow Adam’s salvation gain nothing, except this, that they render themselves heretics and apostates from the truth, and show themselves patrons of the serpent and of death. (AH 3.23.6-8)
Clearly Irenaeus thought the salvation of Adam was of paramount import but why? Might it be that for Irenaeus Adam was not just an individual human being but was, is, humanity itself? Hence to deny Adam’s salvation would be to deny our salvation. If this guess is on the mark, then I wonder about its universalist implications, even though Irenaeus does not appear to have espoused the greater hope.