In 'The Varieties of Religious Experience' (which has an atheist and materialist perspective) William James makes a distinction between Once-born individuals who are naturally well-adjusted souls who do not feel the need to be 'saved' (like Ralph Waldo Emerson, of Walt Whitman), while the Twice-born are those who feel radically miserable, incomplete and who perceive a need to be saved.
I was of the Once-born type for most of my life, and could not understand all the fuss about 'salvation'.
Indeed, I regarded salvation-talk as evidence of an unfortunate illness, or sometimes a thing deliberately induced by manipulation from Churches (who made naturally contented people feel guilty and incomplete in order to get and retain converts).
To the Once-born, sin is perceived as (merely) breaking as a set of (intrinsically imprecise and changeable) social rules; and not a matter of a person's basic perspective on the human condition.
As a Once-born atheist I had been aware of modern man's alienating perspective for some years (for example in this essay: http://www.hedweb.com/bgcharlton/animism.html ) which I interpreted as a consequence of the move away from a hunter-gatherer life that was natural to the evolved human.
But I did not interpret alienation as sin; merely as a source of misery; the response to which was to adjust human emotions by management of lifestyle, use of modern pharmacology, and ultimately genetic engineering: in other words I took a 'transhumanist' perspective.
This survived into my early days as a nominal Christian.
When I became nominally Christian I remained of the Once-born type for many months, until I began to understand the nature of Sin.
So, after a while (after practicing Christianity, as best I understood it, for some months) I began to realize that one cannot really be a Once-born Christian: there is no such thing.
One must be 'born-again'.
(I was, in fact - despite my protestations, until that point actually a monotheist, but not a trinitarian; indeed I could not understand the point of the trinitarian doctrine.)
I now recognize that Christ cannot 'save' a person who fundamentally feels no need to be saved.
There is an absolute necessity for Christians to have a 'born-again' perspective by which a person recognizes their need for salvation; therefore their state of this-worldly incompleteness.
However, and this is vital, this recognition is inevitably partial and temporary.
(Or rather, the recognition is inevitably partial and temporary except among the Saints; but even Saints do not achieve this perspective instantly and completely, but after many years of prayer and ascetic discipline.)
This is a 'mystical' perspective of sin - different from the common 'legalistic' perception of sin in terms of breaking God's Laws.
A basic problem for Christianity today is that we live in a Once-born culture.
Modern secular culture perceives the matter of Christianity's trying to bring people to a recognition of their intrinsic state of sin, as if it were a matter of trying to make happy and well-adjusted people into miserable and needy people.
The common legalistic definition of sin and salvation does not work for Once-born modern mainstream secular culture. It is not really a matter of incorrect, but the legalistic definition does not work.
Legalism does not work in modernity because it does not bring the majority of Once-born people to a recognition of their need for salvation.
Modern secular culture 'understands' - from experience - that 'law' is something labile and - apparently - almost unlimited in its change, evolution, flexibility. Indeed law has been perhaps the primary means of secularization in late modern societies.
Therefore, to moderns, the problem of sin is one which is cured by changing the laws - by re-defining sin - and not by divine intervention.
And this is why I believe that we moderns need a mystical understanding of sin as a false perspective: sin as a false understanding of the human condition.