On July 3, 529, Caesarius of Arles presided over a synod in southern France that was intended to promote the teachings of his favorite theologian, Augustine of Hippo (354-430). Caesarius thought that his fellow church leaders in France had slipped from rigorous Augustinian orthodoxy toward the ideas of Augustine's most bitter enemy, Pelagius. The Synod of Orange was supposed to right the ship and dash Pelagianism once and for all. It didn't.
Augustine was notoriously pessimistic about the human capacity for good. He believed that original sin irreparably tainted all people, so that the only good they could ever do was the good God chose to do through them. He also believed that humans lack the sense or strength to turn to God. Thus it's entirely up to God to save whomever he elects (the doctrine of predestination).
Pelagius didn't like where this thinking led. He particularly objected to Augustine's prayer, "Command what you will, and give what you command." If good will and good action come only from God, Pelagius wondered, are people literally good for nothing? And won't people who are taught that they can't do anything right anyway respond with bad behavior? To preserve morality, and to give humanity some purpose, Pelagius taught that salvation and the Christian life require good use of human free will. He also denied both original sin and predestination.
After 25 years of verbal warfare, Augustine succeeded in getting Pelagianism condemned at the 431 Council of Ephesus. Pelagius, who had never been as "Pelagian" as his followers, had already retreated to a nomad's life in the East. The theology, though, lingered on as "Semi-Pelagianism." As David Allen wrote for Christian History's recent issue on Augustine:
In Provence, ...