Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
When the storm of persecution ceased, the faithful Christians, who, during the time of danger, had hidden themselves in woods and deserts and secret caves, came forth and rebuilt the churches which had been levelled to the ground; founded, erected, and finished the cathedrals raised in honour of the holy martyrs, and, as if displaying their conquering standards in all places, celebrated festivals and performed their sacred rites with pure hearts and lips. This peace continued in the Christian churches of Britain until the time of the Arian madness, which, having corrupted the whole world, infected this island also, so far removed from the rest of the world, with the poison of its error; and when once a way was opened across the sea for that plague, straightway all the taint of every heresy fell upon the island, ever desirous to hear some new thing, and never holding firm to any sure belief.
At this time Constantius, who, whilst Diocletian was alive, governed Gaul and Spain, a man of great clemency and urbanity, died in Britain. This man left his son Constantine [Constantine the Great] born of Helena, his concubine, emperor of the Gauls. Eutropius writes that Constantine, being created emperor in Britain, succeeded his father in the sovereignty. In his time the Arian heresy broke out, and although it was exposed and condemned in the Council of Nicaea, nevertheless, the deadly poison of its evil spread, as has been said, to the Churches in the islands, as well as to those of the rest of the world.