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I think it likely that the concept of Original Sin derived from the philosophical insistence on the absolute/ infinite/ abstract 'Omni' definition of God - omnipotent, omniscient, omnipresent - which attempted to fuse Christianity with Greek-Roman Classical Philosophy.
The Fall is described in Genesis in the Bible, but not the concept of Original Sin - this is a philosophical inference, a second-order interpretation.
Simply, Original Sin has it that since the Fall, all men are born in a state of sin - and it is this situation which Christ's Atonement was needed to remedy.
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I think that once God had been defined philosophically in terms of Omni properties, and also as having created everything (except Himself) from literally nothing (ex nihilo) then the most pressing question for these early intellectual Christians was to explain the function of Christ.
Because a God that could do anything would seem not to need to go through this elaborate, messy and contingent business of Himself being incarnated into mortal life, dying, being resurrected and ascending to Heaven.
I think that Original Sin was intended to explain the function of Christ in the context of a God that could do anything. The idea being that God made everything perfect, but Man exercised his God-given free will to sin in the Fall; and this wrecked creation permanently - requiring a further act of God in Christ's incarnation (with an 'equation' or contrasted equivalence made between Adam and Christ).
But, it has, over the centuries, proved difficult/ impossible to hold-the-line on Original Sin - because the idea of inborn sin implies we are tainted at source, all humans are fundamentally wicked (or else we would not have required a Saviour), and this negative evaluation tends to spread and contaminate everything that humans feel, think and do.
Indeed, belief-in (living-by) Original Sin tends to paralyse the will, because it puts us at odds with God, and it removes any level of self trust, or any confidence in our own evaluations - we cannot (it seems) even evaluate which is a real or valid church - we cannot even get started on our Christian life, because our essence is wrong.
The effect of Original Sin is almost like philosophical relativism - in the sense that in relativism the only core possible belief is that all truths are relative and all other truths are made uncertain; whereas with original sin the only core belief is that we are fundamentally sinners, and all other beliefs are made uncertain by that.
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The Restored Gospel of Mormonism goes right back to first philosophical principles, and rejects the concept of God as primarily an abstract and philosophical 'omni' deity creating everything from nothing - and replaces it with the primary concept, from the Old Testament, of God as a primarily a person: primarily our Heavenly Father, and we as his children. This relational definition is made primary, and any philosophical definitions must fit within this primary 'metaphor'.
Because God is not Omni, and therefore works within the universe, then it is reasonable to assume that the work of Christ was necessary - it was the only (or best) way that God could achieve his purposes.
God could not just make everything as he wanted, by an instantaneous act of will; rather God must work within time and within the matter and laws of the universe, to help his children whom he loves.
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So the Fall happened, and had a permanent effect on possibilities and arrangements - but there is no such thing as Original Sin and modern men and women are born as innocent as were Adam and Eve - but born into a different and corrupted world, and a world of birth and death.
On the other hand, the dawn of consciousness, the reality of birth and death and need to overcome the corruptions of the world offer vast possibilities for spiritual development which would have been impossible without the Fall.
Because our God is not an Omni-God who can do anything instantly, means that the universe sets constraints on action - things can only be done in some ways and not other ways. The Atonement of Christ can be seen as necessary, as the only way that God could achieve his hopes for Man.
The complex 'rigmarole' of having the maker and ruler of this world Christ born into this world as an incarnate Man, his life and teachings, his death by crucifixion, his resurrection and ascension - all these can be understood as a necessary mechanism for our spiritual progression towards godhood, as Sons and Daughters of God to become more like God, more like Jesus Christ.
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My basic point is that the dark and often paralysing concept of Original Sin is only made necessary to explaining the work of Christ by the primacy of an abstract and philosophical Omni concept of God.
But when God is seen as our Father, then Original Sin is seen as firstly unnecessary, and secondly as tending to be pernicious - so it can be and is discarded.
The way is then open for a simple and non-paradoxical understanding of the human condition which has men and women as very mixed but fundamentally good (more-or-less prone to corruption - but not fundamentally and intrinsically sinful); and Christ's work as giving us salvation by (in some way) living the path to eternal resurrected life.
Therefore, without Original Sin, we have all already been-saved by Christ for eternal resurrected life in (some degree of) happiness; so long as we do not choose positively-to-reject that salvation.
For those who accept Christ's salvation; the emphasis of mortal Christian life moves onto exaltation, theosis, sanctification, the process of becoming more god-like - or divinization.
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So, the contingent decision to assimilate the common sense, personal, and family based ancient Hebrew and Christian narrative into the pre-existing Greek/ Roman abstract philosophical understanding can be seen as having had a profound and lasting effect upon Christianity as originally demonstrated and taught by Christ and the Apostles.
The Mormon Restoration goes back to before this philosophical re-conceptualization and reveals what seems to be a simpler, clearer, more positive, optimistic and motivating understanding of the human condition and of Christ's work in transforming it: I believe it is a better way of understanding the difference that Christ made.
Having said that, it is of course perfectly possible to be a real and good and devout Christian with what I would regard as an inferior conceptual understanding of Christianity - indeed that is and always has been the usual situation; not least because most Christians most of the time pretty much ignore the metaphysics and philosophy of their Christianity - and in practice they fit their abstract understanding into their primary personal relationship with God the Father and his Son Jesus Christ.
Mormonism simply takes this 'plain man's' Christianity of loving relationships as its official theology - and there is no need for the doctrine of Original Sin, and little benefit from it (some benefits, but little), and considerable possibility of harm from it.
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But it only makes sense to discard Original Sin if first you discard the Omni concept of God - otherwise you will end-up without any necessary function for Christ, and then Christ becomes an 'optional extra' as in Liberal Christianity - and then, with an optional Christ, the way is open to people, en masse, choosing actively to reject the salvation which comes only by Christ.
Which is the current situation. People who cannot accept Original Sin end up rejecting Christ - but usually unaware that there is a strong and coherent 'third way' that combines full recognition of Christ as our Saviour but without need for Original Sin: which is Mormonism.
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