That humanity is saved by grace all Christians agree, yet once having said that, St Irenaeus believes that we need to say something more:
It was necessary, therefore, that the Lord, coming to the lost sheep, and making recapitulation of so comprehensive a dispensation, and seeking after His own handiwork, should save that very man who had been created after His image and likeness, that is, Adam, filling up the times of His condemnation, which had been incurred through disobedience, — [times] “which the Father had placed in His own power.” [This was necessary,] too, inasmuch as the whole economy of salvation regarding man came to pass according to the good pleasure of the Father, in order that God might not be conquered, nor His wisdom lessened, [in the estimation of His creatures.] For if man, who had been created by God that he might live, after losing life, through being injured by the serpent that had corrupted him, should not any more return to life, but should be utterly [and for ever] abandoned to death, God would [in that case] have been conquered, and the wickedness of the serpent would have prevailed over the will of God. But inasmuch as God is invincible and long-suffering, He did indeed show Himself to be long-suffering in the matter of the correction of man and the probation of all, as I have already observed; and by means of the second man did He bind the strong man, and spoiled his goods, and abolished death, vivifying that man who had been in a state of death. For at the first Adam became a vessel in his (Satan’s) possession, whom he did also hold under his power, that is, by bringing sin on him iniquitously, and under color of immortality entailing death upon him. For, while promising that they should be as gods, which was in no way possible for him to be, he wrought death in them: wherefore he who had led man captive, was justly captured in his turn by God; but man, who had been led captive, was loosed from the bonds of condemnation. (AH 3.23.1)
“It was necessary …” What is the sense of this “necessary”? I wonder what the Latin word is and whether knowing that might help. Irenaeus is surely not suggesting any constraint upon God. I wonder if his meaning might be closer to the sense of fittingness. Once we in Christ that God is love, then it would be unfitting for God to abandon Adam and to not fulfill the purposes of creation. Satan must not be allowed to have the last word. Love brings its own constraints. This is not a matter of mere speculation. Our reflection is grounded upon the knowledge of the Son’s death and resurrection. We begin with the gift of salvation in Jesus Christ.
Irenaeus then compares the sins of Adam and his son Cain:
[These act] as Cain [did, who], when he was counseled by God to keep quiet, because he had not made an equitable division of that share to which his brother was entitled, but with envy and malice thought that he could domineer over him, not only did not acquiesce, but even added sin to sin, indicating his state of mind by his action. For what he had planned, that did he also put in practice: he tyrannized over and slew him; God subjecting the just to the unjust, that the former might be proved as the just one by the things which he suffered, and the latter detected as the unjust by those which he perpetrated. And he was not softened even by this, nor did he stop short with that evil deed; but being asked where his brother was, he said, “I know not; am I my brother’s keeper?” extending and aggravating [his] wickedness by his answer. For if it is wicked to slay a brother, much worse is it thus insolently and irreverently to reply to the omniscient God as if he could battle Him. And for this he did himself bear a curse about with him, because he gratuitously brought an offering of sin, having had no reverence for God, nor being put to confusion by the act of fratricide.
The case of Adam, however, had no analogy with this, but was altogether different. For, having been beguiled by another under the pretext of immortality, he is immediately seized with terror, and hides himself; not as if he were able to escape from God; but, in a state of confusion at having transgressed His command, he feels unworthy to appear before and to hold converse with God. Now, “the fear of the Lord is the beginning of wisdom;” the sense of sin leads to repentance, and God bestows His compassion upon those who are penitent. For [Adam] showed his repentance by his conduct, through means of the girdle [which he used], covering himself with fig-leaves, while there were many other leaves, which would have irritated his body in a less degree. He, however, adopted a dress conformable to his disobedience, being awed by the fear of God; and resisting the erring, the lustful propensity of his flesh (since he had lost his natural disposition and child-like mind, and had come to the knowledge of evil things), he girded a bridle of continence upon himself and his wife, fearing God, and waiting for His coming, and indicating, as it were, some such thing [as follows]: Inasmuch as, he says, I have by disobedience lost that robe of sanctity which I had from the Spirit, I do now also acknowledge that I am deserving of a covering of this nature, which affords no gratification, but which gnaws and frets the body. And he would no doubt have retained this clothing for ever, thus humbling himself, if God, who is merciful, had not clothed them with tunics of skins instead of fig-leaves. For this purpose, too, He interrogates them, that the blame might light upon the woman; and again, He interrogates her, that she might convey the blame to the serpent. For she related what had occurred. “The serpent,” says she, “beguiled me, and I did eat.” But He put no question to the serpent; for He knew that he had been the prime mover in the guilty deed; but He pronounced the curse upon him in the first instance, that it might fall upon man with a mitigated rebuke. For God detested him who had led man astray, but by degrees, and little by little, He showed compassion to him who had been beguiled. (AH 3.24-5)
Irenaeus judges Cain’s sin more severely than Adam’s. Cain slays Abel in full awareness of what he is doing, but Adam is tricked and deceived by Satan. This is a consideration that would be important in a court of law, but how important is it when we are talking about humanity’s expulsion from Eden? Is Cain’s sin less forgivable than Adam’s?