In 418, or immediately afterwards, some British-born advocates of Pelagian teaching – and possibly Pelagius himself – returned to their homeland in the wake of an imperial interdict. Within eleven years they contrived to establish a separate ‘Pelagian Church’ in Britain, which, in the eyes of their opponents, threatened to undermine British Christianity everywhere. This Church succeeded in building on ‘pre-Pelagian’ sympathies, and radicalising them in opposition to ‘the heresy of Augustine’. In 429, Pope Celestine sent Germanus of Auxerre on a mission to Britain to suppress the movement. However, despite claimed successes, Germanus did not manage to eradicate it, as there was another ‘outbreak’ in the 440s. In 431, the same pope sent the deacon Palladius to Ireland as the first bishop to the island’s Christian community, arguably with the intention of checking any possible occurrence of the heresy there. Later in the fifth century, probably beginning in the early 460s, the Briton Patrick conducted an unsanctioned mission to convert the pagan Irish. His writings show that he was engaged against Pelagian ideas, and it is possible that his superiors, whose accusations he was forced to answer, were of Pelagian sympathies. Towards the middle of the sixth century Gildas remarked on the presence of a group in Britain who displayed Pelagian tendencies, and late evidence tells us that the Pelagians were not finally suppressed in Britain until some time in the sixth century, but that this required two synods. Around 640, a pope-elect charged that Pelagianism had been revived in Ireland.

The Pelagian movement defined the common Celtic Church, which we have envisioned as a set of commonalities of theology and of some features of practice in the British and Irish Churches down to the second quarter of the seventh century. Pelagianism may have dominated this Church for only a short time, but its ideology persisted, in some form, beyond the dissolution of the common Celtic Church in the early part of the seventh century. Its adherents taught the natural goodness of man, that a sinless life was possible not only for the Jewish patriarchs but for gentiles as well, that sin was not transmitted through the blood-line, that grace was not necessary for salvation, that God predestined no one, that all men could be saved if they believed, that salvation was achieved through perfect obedience to the law, and that obedience to the law was fostered by asceticism.