We often hear of Pelagianism, or of Pelagius himself. We know it is a heresy which Augustine in the 5th century combated; but we don’t often hear exactly what Pelagianism entails. I thought in an effort to remedy this type of lacuna, at least for those who don’t know, that I would share something from JND Kelly on Pelagius, and in brief, what the main aspect of his troubling teaching entails. Kelly writes:
Pelagius was primarily a moralist, concerned for right conduct and shocked by what he considered demoralizingly pessimistic views of what could be expected of human nature. The assumption that man could not help sinning seemed to him an insult to his Creator. Augustine’s prayer, ‘Give what Thou commandest, and command what Thou wilt’ (da quod iubes et iube quod vis), particularly distressed him, for it seemed to suggest that men were puppets wholly determined by the movements of divine grace. In reaction to this the keystone of his whole system is the idea of unconditional free will and responsibility. In creating man God did not subject him, like other creatures, to the law of nature, but gave him the unique privilege of being able to accomplish the divine will by his own choice. He set life and death before him, bidding him choose life (Deut. 30, 19), but leaving the final decision to his free will. Thus it depends on the man himself whether he acts rightly or wrongly: the possibility of freely choosing the good entails the possibility of choosing evil. There are, he argues, three features in action—the power (posse), the will (velle), and the realization (esse). The first of these comes exclusively from God, but the other two belong to us; hence, according as we act, we merit praise or blame. It would be wrong to infer, however, that he regarded this autonomy as somehow withdrawing man from the purview of God’s sovereignty. Whatever his followers may have said, Pelagius himself made no such claim. On the contrary, along with his belief in free will he has the conception of a divine law proclaiming to men what they ought to do and setting the prospect of supernatural rewards and pains before them. If a man enjoys the freedom of choice, it is by the express bounty of his Creator, and he ought to use it for the ends which He prescribes.
Augustine famously opposed this with his development not only of sin as privatio (privation), but also concupiscence (self-love). But beyond that, if you have ever wondered about Pelagius, or more pointedly about his teaching which has become known as Pelagianism, then this should at least give you a good start. If you want to see what Kelly says further about Pelagius I recommend you pick up his excellent book where he covers this, among other important developments in the early period of the church.
I think all Christians, whether classical Calvinist, classical Arminian, Evangelical Calvinist, Barthian, Lutheran, or what have you share common ground in their opposition towards Pelagianism. Sometimes it requires heresy in order for orthodoxy to be sharpened and articulated in such a way that it provides a fruitful way forward for the church. In this case what Augustine offered against Pelagius served as the basis for what many Christians, even today, think of Pelagianism, and more importantly, how Christians conceive of grace (of course we’ve had other developments since Augustine and Pelagius as well).
 JND Kelly, Early Christian Doctrines. Revised Edition (San Francisco: HarperSanFrancisco, 1978), 356-57.