a 5th-century Christian heresy taught by Pelagius (q.v.) and his followers that stressed the essential goodness of human nature and the freedom of the human will. Pelagius was concerned about the slack moral standards among Christians, and he hoped to improve their conduct by his teachings. Rejecting the arguments of those who claimed that they sinned because of human weakness, he insisted that God made human beings free to choose between good and evil and that sin is a voluntary act committed by a person against God's law. Celestius, a disciple of Pelagius, denied the church's doctrine of original sin and the necessity of infant Baptism.

Pelagianism was opposed by Augustine (Augustine, Saint), bishop of Hippo, who asserted that human beings could not attain righteousness by their own efforts and were totally dependent upon the grace of God. Condemned by two councils of African bishops in 416, and again at Carthage in 418, Pelagius and Celestius were finally excommunicated in 418; Pelagius' later fate is unknown.

The controversy, however, was not over. Julian Of Eclanum continued to assert the Pelagian view and engaged Augustine in literary polemic until the latter's death in 430. Julian himself was finally condemned, with the rest of the Pelagian party, at the Council of Ephesus (Ephesus, councils of) in 431. Another heresy, known as semi-Pelagianism (q.v.), flourished in southern Gaul until it was finally condemned at the second Council of Orange in 529.