When Augustine became a priest in Hippo, a monk from Britain named Morgan, or in Latin Pelagius (which means "islander" -- consider the words "pelagic" and "archipelago"), began to preach in Rome. He denounced what he saw as a reduction of moral standards. His thought was often fundamentalist and authoritarian, and would lead to a austere church of small membership "without spot or stain", i.e., comprising only of the virtuous.
The controversy between Pelagius and Augustine began when this British monk opposed in Rome the famous prayer by Augustine: "Grant what Thou commandest, and command what Thou dost desire." (Confessions 10, 29) Augustine had just completed his challenge to the schismatic Donatists, who had claimed to be the true church, and now here were the Pelagians proclaiming a church that would exclude most of the current membership.
Pelagius was recoiling in horror from the idea that a divine gift (grace - gratia in Latin) is necessary to perform what God commands. For Pelagius and his followers responsibility always implies ability. If the human species has the moral responsibility to obey the law of God, it must also have the moral ability to do it. To the Pelagian, an individual was totally responsible for his sin, as if every sin was done as a deliberate act of contempt against God.
In Pelagianism, Augustine for the first time was confronted by opponents of the same general theological calibre as himself, before a readership capable of judging the arguments of both parties on their purely intellectual merits. Although Augustine was then about fifty-seven years of age (which statistically was "old age" in that era and society) and also past his intellectual prime before this twenty-year debate saw the occurrence of his death, Augustine gave the literary debate against Pelagianism much of his time.
Pelagius saw Christians using human frailty as an absolving excuse for their imperfect living of the Christian life. His response was: "That is nonsense. God has given you free will. You can choose to follow the example of Adam, or you can choose to follow the example of Christ. God has given everyone the grace he needs to be good. If you are not good, you simply need to try harder."
But Augustine introduced the matter of original sin, and Pelagius intimated that there was no such thing. Augustine asked him why, then, was it the universal custom of the Church to baptize infants, and Pelagius had no answer to offer. Augustine saw the teaching of Pelagius as totally undermining the doctrine that God is the ultimate source of all good. It encouraged the virtuous and well-behaved Christian to feel that he had earned the approval of God by his own efforts. The heart of the debate, therefore, centred on the doctrine of original sin, particularly with respect to the question of the extent to which the will of fallen humankind is "free".
As with so many historical arguments, reconstructing exactly what each thought can be rather hard: Pelagius views were misrepresented by his opponents, Augustine's views develop as the debates become fiercer, but Pelagianism has come to mean – unfairly to its founder - the view that human beings can earn salvation by their own efforts. The Pelagian controversy occupied Augustine from at least the year 412 onwards. In this way Pelagianism required his attention for the second half of his life, and some say pessimistically that it is an issue has never really been definitively laid to rest by the Church.
Carthage in North Africa, where Pelagius had sought refuge for a year after the taking of Rome by Alaric in the year 410, was the principal centre of the first Pelagian disturbances. As a regular visitor to Carthage, Augustine could not ignore this difficulty. Pelagius had been disturbed by the low moral tone he had found among Christians in Rome when he had arrived there some thirty years before, and now, in Carthage, he advocated a stricter morality for all Christians. Pelagius and those who agreed with him believed that people could make choices between good and evil, and that, rather than being born sinful, people had no excuse for sinful behaviour, hence every sin was a deliberate act of hatred for God. As early as the year 412 a council held at Carthage condemned the Pelagians for their attacks upon the doctrine of original sin.
The main anti-Pelagian writings by Augustine came at two different periods of his life. On Nature and Grace and On the Proceedings of Pelagius both date from 415 - 416 and constitute two of the most extensive treatments by Augustine of the actual words of Pelagius. Therein Augustine strongly affirms the existence of original sin, the need for infant baptism, the impossibility of a sinless life without Christ, and the necessity of Christ's grace. Augustine's works are intended in part for the common people and for this reason do not address Pelagius by name. A second period of Pelagian intrigues developed at Rome. Pope Zosimus was for a while deluded by a Pelagian named Celestius in Rome. But once Augustine enlightened Zosimus, the Pope pronounced a solemn condemnation of these heretics in 418. Towards the year 426 there emerged a school which afterwards acquired the name of Semi-Pelagian. Its first members were monks of Hadrumetum in Africa, who were followed by others from Marseilles, led by John Cassian, the celebrated abbot of Saint-Victor in France.
Unable to admit the absolute gratuitousness of predestination, they sought a middle course between Augustine and Pelagius, and maintained that grace must be given to those who merit it and denied to others; hence goodwill has the precedence, it desires, it asks, and God rewards. Informed of these views by Prosper of Aquitaine, Augustine wrote On the Predestination of the Saints and On the Gift of Perseverance, probably in the year 428. (This was a magnificent effort, for Augustine was then in his seventy-fourth year, and two years from death.)
LinkAugustine and the Pelegian controversy. A detailed coverage by Mark Ritchie. http://www.ritchies.net/p2wk6.htm