Its story
Pelagius, a lay monk from Britain, taught in Rome from the late 4th century as a spiritual guide to aristocratic families. He was troubled by the moral laxity of many Christians and by the excuse that 'nature is weak'. He insisted that God does not command the impossible: if he commands us to be perfect, we can be. Grace, for him, consisted chiefly in the gift of nature and free will, the law, baptismal forgiveness and Christ's example.
His disciple Caelestius went further: Adam's sin harmed only Adam, infants are born in the state of Adam before the fall, and infant baptism is not for the forgiveness of sins. After the sack of Rome (410) both moved to Africa and Palestine. A synod at Carthage condemned Caelestius (411), but a synod at Diospolis (Palestine, 415) acquitted Pelagius after his explanations.
Augustine of Hippo replied in a long series of works (On Nature and Grace, On the Spirit and the Letter, and later against Julian of Eclanum). For him all humanity is wounded by original sin, and the will, without God's inner grace setting it free, cannot love the good in a saving way. The Council of Carthage of 418 condemned Pelagian theses in canons later gathered into the African code (419); Pope Zosimus, after hesitating, condemned them in his Tractoria (418), and the Council of Ephesus (431) condemned Caelestius. Jerome also wrote against Pelagius.
As with other heresies, Pelagius is known partly through his opponents, though several of his works survive (such as his Commentary on Paul's letters and his Letter to Demetrias). Some historians argue that 'Pelagianism' was partly a polemical construct grouping diverse ideas. Ever since, the charge of 'Pelagianism' has been a frequent weapon in disputes over grace, including between Catholics and Protestants, and each tradition draws the line somewhat differently.