What parent thinks it is best to raise a child to forever need to rely on their parents? What parent thinks the best way to raise a child is by first teaching the child they are flawed and sinful, but by devoting themselves to loving and worshiping them alone, the parent promises to make that child better? And yet, this is exactly what St. Augustine is applauded for doing to Christians.
Basically, St. Augustine argued that everyone bares the stain of Adam & Eve's "original sin" of disobedience to God, in the Garden of Eden. Thankfully, humanity was able to obtain forgiveness for that "sin" by savagely murdering God himself, in the form of Jesus, and by doing so, all was made right with the world (except for all of the brutal wars and sin that has raged around the world ever since - apparently the "Jesus fix" didn't take so well).
Unlike St. Augustine's belief that everyone was born with the stain of original sin (that God, like a doctor being stingy with his cures, choose to withhold partially from Mary and totally from Jesus - but NO ONE ELSE!) Pelgaius disagreed, and felt that people were not encumbered by the sins of those who came before them. Pelgaius's view was defended by his disciple Caelestius and finally Julian of Eclanum, whom Augustine battled with theologically up to his death in 430 A. D.
The Original Sin of St. Augustine
What St. Augustine was really battling, however, was not Plegaius, Caelestius, or Julian of Eclanum, but his own sins in the brothels of Rome.
St. Augustine had spent his youth whoring around Rome, where there were brothels aplenty, only to become so disgusted with himself because of his unbridled sexual exploits that he became convinced that no one else should have to suffer the same pleasures that he had subjected himself too.
As such, his entire theology is then informed by his own disgust for his own "original sin," which convinced him that there must be something in him that predisposes him to enjoying the pleasures of the flesh, that he had only so recently decided he had grown sick of. That predisposition must be "original sin," and not his own raging hormones.
This concept only ensured that people would need to depend on God, and more importantly, the worldly and wise Catholic Church (i.e. the new Sanhedrin) for the rest of their miserable lives. And all for simply a chance - just the lottery of a chance - that by obeying what this earthly Church commands, they will please God enough to NOT be thrown forever into a lake of fire.
Yet fostering such complete emotional and intellectual dependence on a "Church" to cure and save people from a "disease" called "original sin" only empowers the Church at the expense of all those who, like St. Augustine, are unable and unwillingly to accept full responsibility for their own actions on the one hand, and that not everyone is exactly like St. Augustine (who was so clearly unable to control his sexual urges, and mostly because of his own raging hormones, that he decided women were were as bad as the devil) on the other.