Pictured above (supposedly) are Pelagius, founder of the Pelagian heresy, and Augustine of Hippo; one a heretic and one a saint. Sadly, I'm not sure which is Pelagius and which is Augustine.
We don't know much about the life of Pelagius but for the fact that he was a monk, and was considered to be British, or Irish, or Scottish depending on the record on which you rely. He and Augustine were contemporaries, both born in the 4th century C.E. or A.D., whichever you prefer, both dying in the 5th century. Augustine, of course, became a bishop.
While we don't know much about the life of Pelagius, we know far too much of the life of Augustine, or so I think. We know much of Augustine's life because he thoughtfully told us of it, in great detail, in his Confessions. That work describes his sinful, profligate ways before his fittingly dramatic conversion to Christianity and attendant salvation, but also includes prayers of thanksgiving and speculations on whatever theological or philosophical issues caught his fancy.
Presumably, Augustine's Confessions served as a model for the Confessions of the similarly self-regarding Rousseau, who also carefully cataloged his misdeeds for our benefit though not, it seems, for any religious reasons. Nevertheless, Rousseau was also saved, but not by God, and his misdeeds were evidently rendered tolerable when he became the Rousseau he thought we should know and admire.
Both Augustine and Rousseau were apparently the kind of men who relished describing their sins. Perhaps Augustine flaunted his sinfulness in part due to his acceptance and it may be said creation of the doctrine of Original Sin, that peculiar belief that all of humanity is, forever, tainted by the sin of Adam and Eve and so damned unless redeemed by God's grace, which is in God's gift. This is not a doctrine Pelagius was willing to accept, but which the Church ultimately did accept. Thus, Pelagius became a heretic and Augustine a saint.
Unfortunately, the writings of Pelagius were lost, or more probably were destroyed after his condemnation. So we know them only through the writings of those who thought he should be condemned. It seems though that Pelagius thought human nature to be untouched by the sin of Adam and Eve. As a consequence, humans had the capacity to be good and to renounce evil, and didn't require the gift of God's grace to do so. Immorality was a matter of will, or lack of will to do good. People could do good deeds without the intervention of God. Human will in itself, as created by God, was sufficient to assure a sinless life. God may help us, inspire us to do good, but he doesn't cause us to do good by granting us his grace.
For Pelagius and his followers, the notion that human nature is prone to sinfulness from birth excuses immoral conduct. It serves as an explanation of our evil deeds as the result of something beyond our control; we're naturally bad because Adam and Eve did what they did or failed to do what they failed to do. How are we to be blamed for doing what we have to do by our nature, i.e. for not being able to do the impossible?
I find it interesting that, for me at least, certain Christian heresies seem so much more sensible, so much more edifying, so much more ennobling than orthodox Christian doctrine. Pelagianism is one of those heresies, as it teaches that we're responsible for our own conduct and are not damned by the actions of mythological forbears, a damnation which seems extraordinarily vengeful coming from the God of the universe. The Arian heresy which avoids the problems encountered by insisting Jesus was wholly God and wholly man at the same time, consubstantial with the Father, is to me another of those heresies.
I also find it interesting that Pelagianism seems similar in various ways to the teachings of the pagan philosophers, and to the teachings of the Stoics in particular. Pelagius thought that as creature of God we cannot be inherently sinful and debased; the Stoics thought us good, not evil, and partakers in the divine. The Stoics emphasized our ability to act for the good, "in accordance with nature", nature itself being good, and that bad conduct is the result of a failure in our use of reason, resulting from our desires and fears as to things beyond our control. Pelagius and his followers taught we could be good by the exercise of our own will. According to the Stoics, no God or gods were needed in order for us to act in accord with nature. God's grace wasn't necessary to a good life according to Pelagius.
Christianity borrowed freely from the pagan philosophers in various ways. What was it about Pelagianism that merited its condemnation by the Church, and still merits its condemnation? It seems that it's felt that Pelagianism doesn't accept that we're ultimately dependent on God's mercy and indulgence to be saved, and incapable of determining what is good by ourselves. It seems an unworthy basis on which to condemn.