What was religion of the British Isles in the fourth century?

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In the 4th century, it is likely that the religion of the British Isles would have been a mixture of both Christianity, and indigenous and Roman paganism.
The southern part of Britain had been a Roman province since the first century CE and appeared thoroughly Romanised. However, Britain seceded from the Empire in 287 and was reconquered in 306. The principal religion of the people was now Roman paganism. However there must have been a significant Christian presence during the reign of Emperor Constantine, as it seems that pagan sites were liable to plundering attacks from Christians. Under Constantine and his Christian successors, Christianity flourished in Britain, as elsewhere, but it was not until around 429 that the ruling class of Britain was entirely Christian.

The Welsh, Scots and Irish inhabitants still followed their traditional Celtic religion during the fourth century. St Patrick is said to have established Christianity in Ireland in 448.