Early Christianity in Celtic Britain

Britain & Ireland in the mid-5th century CE,
between the Roman withdrawal and the founding of Anglo-Saxon kingdoms
Brythonic Goidelic Pictish areas
wikimedia

The history of Christianity in Britain can be divided by the time before and after the mission sent by Pope Gregory I the Great (540-604). To some degree it corresponds also with the demographics so that the early phase concurs with the mainly Celtic presence in the British Isles before the Anglo-Saxon invasion and conquest.

A wikipedia article summarizes nicely the early period

Christianity reached Britain by the third century of the Christian era, the first recorded martyrs in Britain being St. Alban and Aaron and Julius, citizens of Carlisle, during the reign of Diocletian.

Gildas dated the faith's arrival to the latter part of the reign of Tiberius.

Christianisation intensified with the legalization of the Christian religion under Constantine in the early 4th century and its promotion by subsequent Christian emperors, but in 407 the Empire withdrew its legions from the province to defend Italy from Visigothic attacks in which the city of Rome would be sacked in 410. The legions did not permanently return to Britain, Roman tax and army influence ended on the isle and, with the decline of Roman imperial political influence, Insular Christianity retained distinct traditions and practices through the era of Church Councils.

Clerics such as Germanus of Auxerre accused some British bishops of the heresy of Pelagianism and sought their removal from office.

According to hagiographies written some centuries later, Illtud and his pupils David, Gildas, Paul Aurelian, Samson and Deiniol from the next generation, were leading figures in sixth-century Britain. Some of them were also active in Brittany.

Others who influenced the development of British Christianity include Dubricius, Cadoc, Petroc, Piran, Ia and Kentigern (also known as Mungo).

A monastery-centred establishment seems to have grown up in sixth-century Britain, though our knowledge of this period there is limited.

There may have been interaction with Ireland at this time, perhaps partly brought about by a very severe plague in Ireland in 548/9, only a few years after the extreme weather events of 535–536.

However, Bede speaks of "the monastery of Bangor, in which, it is said, there was so great a number of monks, that the monastery being divided into seven parts, with a superior set over each, none of those parts contained less than three hundred men, who all lived by the labour of their hands."

At the end of the 6th century, the face of Christianity in Britain was forever changed by the Gregorian mission.
Read the entire wikipedia article here

Roman rule was not totally limited south of Hadrian's wall but native Celtic speakers all over the British Isles must have associated Christianity with the Latin culture and foreign occupation. The summary above suggests that fourth century was an intensive time for the growth of the Kingdom of Heaven in Britain and that the plant of Christianity survived the departure of the legions at the time of the sack of Rome, Saint Augustine and Pelagius.