Irenaeus of Lyons, answered from the texts and cited to the page.
Take the order as I give it. God was able to make man perfect from the first, for all things are possible to Him; but what is newly made is inferior to its Maker by the very fact of later origin, and so comes short of the perfect, being infantile and unexercised. As a mother could give strong food to her infant and does not, since the child cannot yet bear it, so man, being yet an infant, could not receive perfection.1
Hence the Lord Himself came to us as we were able to behold Him, and offered Himself as milk to infants.1 Now Adam. In the Demonstration I say it plainly: man was a child, his understanding not yet perfected, and therefore easily led astray by the deceiver.2 Their nakedness was the innocence of a childlike mind, which could not conceive what wickedness, through lusts and shameful desires, brings to birth in the soul.3
A law was given lest he think too highly of himself as one with no lord; keep it, and he should remain immortal; break it, and he should be dissolved to the earth from which he was taken.4 So I do not make the Fall small. The wages are death, and the penalty is named in the commandment itself. But I will not call it the quarrel of a rebel against the Father, as the Valentinians tell it, where the serpent is Sophia's instrument against Ialdabaoth and the transgression is a stratagem for gaining knowledge of the power above.5
That is a tale of powers contending, and it makes the Creator a jealous and ignorant tyrant. With me the matter is plain: Adam was beguiled by another under the pretext of immortality, and was at once seized with terror. He hid, not as though he could escape God, but in the confusion of one who has transgressed and feels unworthy to hold converse with Him.6
That is a child ashamed. And the sense of sin led him to repentance, for he girded himself with fig-leaves, a covering that gnaws and frets the body, confessing that he had lost the robe of sanctity he had from the Spirit; and God, being merciful, clothed him with tunics of skins instead.6 Mark also how God dealt with the three. He put no question to the serpent, but pronounced the curse on him first, that it might fall upon man with a mitigated rebuke; He detested him who led man astray, and showed compassion by degrees to him who was beguiled.6
As to your second question, you ask whether this is a theology without original sin. I did not set the matter in the terms you use, and I will say only what I wrote. I hold that by the disobedience of one man, moulded from virgin soil, the many were made sinners and forfeited life; and that by the obedience of one man, born of a virgin, the many should be justified and receive salvation.7
Death reigned from Adam, and the Law, being spiritual, made sin stand out but did not destroy it, for sin had dominion not over the Spirit but over man.7 So the remedy had to be a man: the Word made that very thing which man was, that sin should be destroyed by man and man go forth from death.7 That is no education merely, as though the child needed only to grow.
He needed a victor. The Lord fought as man contending for the fathers, bound the strong man, set free the weak, and destroyed sin.7 How the one man's error became the whole race's death beyond this I have not worked out in these books. Yet the patience is real. The Lord passed through every stage of life, restoring to all communion with God.7 And the end of the long nursing is that, fed from the breast of His flesh, we become able to contain the Bread of immortality, which is the Spirit of the Father.1
A God who bears with the infant, and does not abandon the beguiled, is the Maker of whom the heretics said He was either ignorant or unjust. He is neither.
God could have made man perfect at once, for all things are possible to him; man could not have received it, as a newborn cannot take meat. Adam and Eve were made as children, their minds not yet grown to the measure of their freedom, and the serpent deceived them; their disobedience was real and its wages were death, but it was the fault of the inexperienced, and God pitied the deceived and cursed the deceiver.
Since then God has borne with the apostasy to educate man, giving the law as a pedagogue, accustoming man to bear the Spirit and the Spirit to dwell in man, so that man, made, grows, is strengthened, is glorified and sees God. Free will is never taken away.8910
so also it was possible for God Himself to have made man perfect from the first, but man could not receive this [perfection], being as yet an infant.Against Heresies, Book IV
But man was a child, not yet having his understanding perfected; wherefore also he was easily led astray by the deceiver.The Demonstration of the Apostolic Preaching
there was in them an innocent and childlike mind, and it was not possible for them to conceive and understand anything of that which by wickedness through lusts and shameful desires is born in the soul.The Demonstration of the Apostolic Preaching
a law was given to him by God, in order that he might perceive that he had as lord the Lord of all... if he should keep the commandment of God, he should ever remain such as he was, that is to say, immortalThe Demonstration of the Apostolic Preaching
their mother (Sophia) cunningly devised a scheme to seduce Eve and Adam, by means of the serpent, to transgress the command of Ialdabaoth.Against Heresies, Book I
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 GodAgainst Heresies, Book III
as by the disobedience of the one man who was originally moulded from virgin soil, the many were made sinners, and forfeited life; so was it necessary that, by the obedience of one man, who was originally born from a virgin, many should be justified and receive salvation.Against Heresies, Book III
And on this account does Paul declare to the Corinthians, "I have fed you with milk, not with meat, for hitherto ye were not able to bear it." That is, ye have indeed learned the advent of our Lord as a man; nevertheless, because of your infirmity, the Spirit of the Father has not as yet rested upon you. "For when envying and strife," he says, "and dissensions are among you, are ye not carnal, and walk as men?" 3.Against Heresies, Book IV
And so fair and good was this Paradise, that the Word of God continually resorted thither, and walked and talked with the man, figuring beforehand the things that should be in the future, (namely) that He should dwell with him and talk with him, and should be with men, teaching them righteousness.The Demonstration of the Apostolic Preaching
But as our Lord is alone truly Master, so the Son of God is truly good and patient, the Word of God the Father having been made the Son of man. For He fought and conquered; for He was man contending for the fathers, and through obedience doing away with disobedience completely: for He bound the strong man, and set free the weak, and endowed His own handiwork with salvation, by destroying sin.Against Heresies, Book III
The alternative to the Augustinian account that the West did not take, recovered by modern theodicy and by Orthodox theology, and faulted by critics for making the Fall small and the Redeemer's work gradual.