Pelagius

Pelagius (fl. c. 390-418) was an ascetic who opposed the idea of predestination and asserted a strong version of the doctrine of free will. He was accused by Augustine of Hippo and others of denying the need for divine aid in performing good works. For him (according to them), the only grace necessary was the declaration of the law; humans were not wounded by Adam's sin and were perfectly able to fulfill the law apart from any divine aid. He denied the more specific doctrine of original sin as developed by Augustine. Pelagius was declared a heretic by the Council of Carthage. His interpretation of a doctrine of free will became known as Pelagianism. He was well educated, fluent in both Greek and Latin, and learned in theology. He spent time as an ascetic, focusing on practical asceticism, which his teachings clearly reflect. He was certainly well known in Rome, both for the harsh asceticism of his public life as well as the power and persuasiveness of his speech. His reputation in Rome earned him praise early in his career even from such pillars of the Church as Augustine, who referred to him as a "saintly man." However, he was later accused of lying about his own teachings in order to avoid public condemnation. Most of his later life was spent defending his doctrine against Catholic theologians who held that Catholicism came from the apostles and that Pelagius was spreading novelties in the Faith unknown to the apostolic tradition.

Due to his status as a heretic, little of his work has come down to the present day except in the quotes of his opponents. However, more recently some have defended Pelagius as a misunderstood orthodox:

Recent analysis of his thinking suggests that it was, in fact, highly orthodox, following in the tradition established by the early fathers and in keeping with the teaching of the church in both the East and the West. ... From what we are able to piece together from the few sources available... it seems that the Celtic monk held to an orthodox view of the prevenience of God's grace, and did not assert that individuals could achieve salvation purely by their own efforts...

Read more about Pelagius:  Beginnings, Opponents, Pelagius and The Doctrine of Free Will, Death and Later, Writings By Pelagius, Pelagius in Literature and Film