Pelagius spent the most formative period of his life in Rome, where in the text ‘De induratione cordis Pharaonis’ he first expounded the theories that later constituted the Pelagian heresy. He stated his belief that people merit salvation by deliberately choosing of their own free will to observe God's precepts. His teachings denied that mankind was conceived in original sin and that there was need for divine redemption; free will, with God's grace as an external aid, enabled one to achieve the level of moral perfection which Pelagius as an ascetic saw as essential for salvation. While in Rome (406×409), Pelagius wrote his great commentary on the Pauline epistles, which was afterwards much used by Irish scholars. His ability as a thinker, his zeal for moral reform, and his personal asceticism evidently gained him many disciples in Rome, some of whom, most notably Celestius, took his teachings to further extremes.
Following the sack of Rome by the Goths in 410, Pelagius and Celestius travelled to Palestine via Carthage. Between 411 and 418 he came under attack from his arch-enemy Augustine of Hippo, who published his tracts ‘On the merits and forgiveness of sins and infant baptism’ and ‘On the spirit and the letter’, rebutting the Pelagian doctrine on original sin and infant baptism. In 415 Pelagius and Celestius were both accused of heresy by the synods of Diospolis and Jerusalem – though Pelagius was able to acquit himself – and in the following year by two African councils at Carthage and Milevis. Eventually, Pope Innocent I declared Pelagius and Celestius both excommunicate. That same year, Augustine wrote an account of Pelagius and his doctrine, which validated the general condemnation of Christendom. A general council of the African church held at Carthage (418) issued nine definitive canons countering Pelagianism which were then confirmed by Pope Zosimus in his ‘Epistula tractoria’. That same year, an official rescript from the emperor Honorius condemned those who denied the Fall; it ordered the banishment of Pelagius, who was expelled from Jerusalem and Palestine. After staying a short while in Antioch, he finally left for Egypt, where he died some time after 418, possibly as late as 427.
The Pelagian controversy dragged on well into the late fifth century, and in parts of Britain – where the ‘heresy’ may first have taken root as a movement for social reform – into the sixth. Much of the evidence for the man, his work, and the growth of the heresy which he so unwittingly originated, is now lost, the surviving sources mainly representing the gossip and detraction of his enemies. There is no doubt, however, that remnants of Pelagianism or an intellectual interest in the Pelagian writings survived in Gaul, Britain, and Ireland for centuries. The persistence of Pelagianism, perhaps of a very attenuated kind, in Dyfed and west Britain may be connected with the existence of substantial Irish settlements from the fourth century onwards, as attested by ogam and Latin inscriptions and other historical evidence.
It is impossible to assess the extent of interest in Pelagian writings in Ireland before the seventh century. In the Irish schools interest mainly focused on his commentary on the Pauline epistles. The outstanding success of studies by Heinrich Zimmer (qv) and A. Souter into the Irish tradition of Pelagius and the manuscript tradition of his commentary may have stifled further research into the Irish sources of that tradition, which (on the linguistic evidence of the Irish glosses preserved in Würzburg M.p.th.f.12 alone) certainly reaches back to the seventh century. In the eighth and ninth centuries Irish monks and scholars travelling to the Continent took along manuscripts of Pelagius's commentary with its learned bilingual glossing. Three continental libraries, all associated with the Irish mission, claim manuscripts of the Pelagian Pauline commentary. There has been no adequate study of this phenomenon. Nor has any concerted attempt been made to ascertain evidence of Irish use, outside of the gloss-commentary tradition, of the many other works included within the ‘Corpus Pelagianum’, some of which are certainly of Pelagian authorship. The Irish use of Pelagius, early and extensive though it is, need not impel us to speculate, like Heinrich Zimmer, E. G. Bowen, and others, that he was therefore an Irishman.