Pelagianism views humanity as basically good and morally unaffected by the Fall. It denies the imputation of Adam's sin, original sin, total depravity, and substitutionary atonement. It simultaneously views man as fundamentally good and in possession of libertarian free will. With regards to salvation, it teaches that man has the ability in and of himself (apart from divine aid) to obey God and earn eternal salvation. Pelagianism is overwhelmingly incompatible with the Bible and was historically opposed by Augustine (354-430), Bishop of Hippo, leading to its condemnation as a heresy at Council of Carthage in 418 A.D. These condemnations were summarily ratified at the Council of Ephesus (A.D. 431).
Pelagius was a monk from Britain, whose reputation and theology came into prominence after he went to Rome sometime in the 380's A.D. The historic Pelagian theological controversy involved the nature of man and the doctrine of original sin.
Pelagius believed that the consequences of Adam and Eve’s sin (the Fall) were restricted to themselves only; and thereby denied the belief that original sin was passed on (or transferred) to the children of Adam and thus to the human race. Adam's sin merely "set a bad example" for his progeny and Jesus "set a good example" for mankind (thus counteracting Adam's bad example). Pelagianism teaches that human beings are born in a state of innocence with a nature that is as pure as that which Adam was given at his creation.
As a result of his basic assumption, Pelagius taught that man has an unimpaired moral ability to choose that which is spiritually good and possesses the free will, ability, and capacity to do that which is spiritually good. This resulted in a gospel of salvation based on human works. Man could choose to follow the precepts of God and then follow those precepts because he had the power within himself to do so.
The controversy came to a head when Pelagian teaching came into contact with Augustine. Augustine did not deny that man had a will and that he could make choices. But, Augustine recognized that man did not have a free will in moral issues related to God, asserting that the effects original sin were passed to the children of Adam and Eve and that mankind’s nature was thereby corrupted. Man could choose what he desired, but those desires were influenced by his sinful nature and he was unable to refrain from sinning.
Pelagius cleared himself of charges, primarily by hiding his real beliefs; however, at the Council of Carthage in 418 A.D., his teachings were branded as heresy. The Council of Ephesus in 431 A.D., again condemned Pelagian doctrine and it was banished in the Greek portion of the church. However, in the West, the teachings held on, primarily in Britain and Gaul.
Pelagian teaching was replaced with Semi-Pelagianism which sought a middle ground between Pelagianism and Augustinianism, but it too was condemned at the Second Synod of Orange in 529 A.D. However, elements of Semi-Pelagianism continued in the Western (Roman) church. It emerged again after the Reformation in modified form in Arminianism which was rejected by the Reformed churches at the Synod of Dort in 1618-1619 A.D.