Pelagius was born around 354, probably in Britain. Around 380 he arrived in Rome, where, though never ordained, he became an influential spiritual director. A rigorous ascetic, he was appalled by the moral laxity of Roman Christians — and laid the blame on the doctrine of grace, which, he thought, killed in people the will to effort. Hearing a line quoted from Augustine's Confessions — “Give what You command, and command what You will” — he was indignant: if everything depends on God, why strive at all?
Pelagius held three theses. First: Adam was mortal even before the Fall — his sin harmed himself alone and did not alter the nature of the human race. Second: newborns are in the same state as Adam before the Fall; they inherit no original sin. Third: free will is sufficient to choose the good and attain holiness. Grace is useful but not necessary — it merely eases the way. His ally Caelestius put it more sharply: mankind neither dies through Adam's sin nor rises through the resurrection of Christ.
His opponent was Augustine of Hippo, the greatest theologian of the Latin West. After the Fall, Augustine taught, human nature is so deeply damaged that without the grace of God man cannot even desire the good. Salvation is a gift, not a wage. Against Pelagius Augustine wrote De natura et gratia (415), De gratia Christi (418) and a series of further treatises, working out the doctrines of original sin, predestination and irresistible grace.
Two African councils — at Carthage and Milevis (416) — condemned the teaching. Pope Innocent I concurred and excommunicated Pelagius and Caelestius. The Council of Carthage of 418 pronounced Pelagianism a heresy once and for all. Pelagius's traces then vanish — he probably died in Palestine.
A century later came semi-Pelagianism: John Cassian in Marseille. He acknowledged original sin and grace, but held that the beginning of faith — the initium fidei, the first step toward God — depends on free will. The Second Council of Orange, in 529, condemned this too: even the beginning of faith is a gift of grace.
The quarrel between Augustine and Pelagius shaped the whole of Western theology. Augustine's teaching underlies both Catholic and Protestant divinity; Luther and Calvin counted themselves his heirs. But Pelagianism is the heresy that will not die. Every time someone says “I can manage on my own; one simply has to try; everything depends on my effort”, he is repeating Pelagius. Secular humanism, the self-improvement industry, the ideology of the self-made man — Pelagianism without the theological vocabulary.