Two major criticisms of the belief in original sin, according to Alan Jacobs’ essay in the history of the idea (Original Sin), can be summarized like this:
First let me ask a question: Are infants who die prior to baptism sent to hell? Or, do you think infants innocent until they respond to God? And what does this say about original sin? Surely some pastors have been asked this one.

“And so, because a brilliant and devout old bishop [Augustine] could not resist the controversialist’s temptation — to take even a caricature of his views and defend it to the death, rather than show dialectical weakness — the whole doctrine of original sin, in Western Christianity anyway, got inextricably tangled with revulsion toward sexuality and images of tormented infants. And there has never been a full and complete disentangling” (66).
I think he could have offered to us a little clearer prose, but the point is this: Augustine’s stubborn fight with Julian was relentless; and Augustine defended his ideas that sin gets going in sex and original sin means unbaptized infants are damned to hell. So, these ideas — and not original sin itself — these ideas, and how they all got tied together, are what give the doctrine its bad name.
Jacobs took a long jog before this chp began to pick up its pace … some stuff from Milton and CS Lewis and then some stuff on “it’s the woman’s fault!” … and then we get Augustine.
For Augustine, original sin was an intense idea. It permeated everything. He fought Pelagians/Pelagius over it. Jacobs thinks Pelagius was a bit of a motivational speaker. That perfect obedience was obligatory and therefore possible. He thought Augustine’s theory of the corruption of the will was absurd. He sums up Pelagius like this: “Grace empowers us to avoid failure, rather than consoling us after we have failed” (51). Adam, Pelagius said, would have died anyway. Adam’s impact on us is as a bad example. The good news: we can obey; the bad new: we can disobey.
Jacobs deconstructs Pelagius for the possibility of being obedient leads to doubt about our getting the job done, while Augustine’s emphasis is “curiously liberating” (53) since it permits failure and tolerates sin in others.
I jotted this down in the margin: “Many Christians today are Pelagians without his anxiety or fear of judgment, because they’ve covered their anxiety with casual grace.”
Then he turns to Julian of Eclanum, who spent years fighting Augustine from the angle of Pelagianism and whom Augustine fought back with similar invective. It really is an ugly episode. It drove Augustine to say lust, a sinful thing, will accompany all sex; it drove him to think unbaptized infants would be sent to hell.