Jeffrey Burton Russell reviews Original Sin: A Cultural History, by Alan Jacobs:
Jacobs efficiently defends Augustine (AD 354–430) against the many attacks against him as the author of original sin, demonstrating that doctrines of original sin similar to Augustine’s preceded him by at least two centuries in both the East and the West. Jacobs quickly dismisses the still widely held belief that original sin was sexual — Adam and Eve practiced free sex in Eden before their eviction. Original sin is the initial assertion of human pride against God.
Augustine did maintain that original sin, once it existed, was transmitted sexually through generations, in much the same way that today we understand genetic flaws are passed on. Contrary to another common misconception about Augustine, he was adamant that the source of sin does not lie in the body but rather in the corruption of the will. In fact, he spent a great deal of his career denouncing the Manichean belief that the human body is essentially evil.
Source: The Wilson Quarterly
Even if theologians before Augustine were discussing original sin in ways similar to what his own would eventually be, this doesn't really let Augustine off the hook. Just because he wasn't the first to arrive at the idea doesn't mean that he shouldn't be treated as the author of the concept as it would be understood and used in Christian theology.
Jacobs’s most original and provocative argument is that original sin has strong democratic implications. Denial of original sin leads to elitism: Take, for instance, the duchess who simply refuses to believe that she shares a common nature with the unkempt commoners of field and street, or the self-righteous people who believe that they can make themselves good by stacking up a higher pile of good deeds than of bad ones. Their underlying assumption is that some people have exempt status, or higher virtues, or brighter minds, that others lack — plainly speaking, that some people (usually us) are better than other people (them).
Original sin, on the other hand, is egalitarian because it means that everyone is alienated from God and has an innate tendency to sin. Equally egalitarian is the belief that Christ died in order to give everyone the liberty to escape sin. No one person can dare to consider himself or herself better than others, and no nation or race should dare to do so either.
Clearly, though, at least some aristocrats did believe that they didn't share a common nature with the peasants, so whatever pro-democratic influence might be found in original sin in theory, it doesn't necessarily exist in practice. There is something disingenuous about asserting all sorts of wonderful benefits from a religious doctrine when the historical reality is that few adherents of that doctrine were ever actually influenced in manner described. If original sin is so pro-democratic, why didn't democracy come to the Christian west until after the Enlightenment, wars of religion, the growth of secularism, and the removal of ecclesiastical institutions from seats of political power?
Moreover, any pro-democratic influence that might be had from original sin was also clearly overwhelmed by anti-democratic tendencies in Christianity. For example, Christianity does teach that everyone is equal, but only "in Christ," which allowed Christians to justify all sorts of political, social, and economic inequalities in the physical world. If clear, unambiguous declarations of equality can be so easily subverted, then complex arguments for equality drawn from doctrines which make no specific mention of the concept can hardly be expected to have any real impact.
Finally, teaching that everyone is equally bad may indeed entail the premise that everyone is equal, but equality under those conditions isn't exactly a positive addition to democracy. On the contrary, if everyone really is so depraved and awful, why should they all be given sovereign political power in the first place? This can easily be used to justify invest sovereign power in a few elites who, while also suffering from original sin, have the learning and righteousness to enforce the perfect Will of God.