For nine years, Augustine was a Manichean, a devotee of of the teachings of Mani, founder of a Persian moral cult. Like the Gnostics of the first century, Mani and his followers were dualistic, teaching that the flesh was sinful and impure, while the spirit was light and life. As a Manichean, this teaching was a comfort to Augustine, as it let him blame his continued sexual sin on his lower fleshy nature, but still be moral by emphasizing the separateness of flesh and spirit.
Augustine's years with the Manicheans left its impact on the Church, as he brought this teaching into the the Church through his teaching on Original Sin. A. T. Overstreet, in his on-line book, "Are Men Born Sinners?, The Myth of Original Sin," notes:
Augustine's nine years with them [the Maniceans] accustomed him to regard human nature as essentially evil and human freedom as a delusion. Augustine next fell under the influence of Neo-Platonism, and his theological views were strongly influenced by this philosophy as well. However, his doctrine of sin shows the obvious influence of the Gnostic teachings of Manichaeism, in which he assumes the most ridiculous teaching of all the heathen philosophies the teaching that matter can be sinful. And this is the source of his doctrine that sin can be passed on physically from one person to another.
Harnack says:
We have, finally, in Augustine's doctrine of sin a strong Manichaean and Gnostic element; for Augustine never wholly surmounted Manichaeism.
and...
Augustine's doctrine of sin, with his belief in the inherent sinfulness of the physical constitution, is wholly Manichaean. His idea that sin is propagated through the marriage union, that sexual desire is sin and that sexual lust in procreation transmits sin is also Manichaean. Augustine built his doctrine of original sin upon this premise that sexual lust in procreation transmits sin.
