Christianity very much did spread to Britain. The council of Arles in 314 was attended by three British bishops, representing British communities. We are not certain if there was British bishops in the council of Nicaea called by Constantine, but we know there were British bishops in the Council of Serdica in 345 and Council of Arminium in 359. Britain even managed to produce its own unique Christian heresy called Pelagianism that was very popular within Britain and led Saint Augustine to spend some of his time refuting it, so it can be assumed that Christianity was well-established in Britain.

However, Britain was unique in one regard. It was invaded by barbarians that didn’t immediately convert to Christianity. Goths, Franks, Vandals, Burgundians, Suebi, lombards etc converted to Christianity almost immediately upon entering the Roman Empire or even before they entered the Roman Empire although many largely converted to the Arian variety of Christianity. Either way, the Roman Christian writers never referred to the Goths as being ‘pagans’. Instead Goths were always branded as heretics by Christian Roman writers. For instance, the Roman Christian, Orosius, writes with extreme hostility in the early 5th century that:

“The Goths had petitioned through ambassadors that bishops be sent to them from whom they might learn the rule of the Christian faith. In fatal perverseness the emperor Valens sent teachers of the Arian doctrine, and the Goths continued to believe what they first learned concerning the basic principles of the faith. Therefore, by the just judgment of God Himself, Valens was burned alive by the very men who, through his action, will burn hereafter for their heresy.”

Of course the sources we have that come from Romano-British writers refers to the Anglo-Saxon invaders as being unquestionably ‘pagan.’ This is strongly emphasised by the British priest Gildas, so when he extorts the western British lords to drive back the Anglo-Saxon invaders, he does so by referring to the invaders as being pagan. This is clearly an appeal made to what is presumed to be a largely Christian audience (the lords of western Britain), so we can also assume that upper classes would have been Christianised by the time of the Anglo-Saxon invasions.

But why didn’t Anglo-Saxons immediately convert to Christianity like other barbarians who had migrated to former Roman provinces? It can’t be said with any certainty, but it might simply because no one had bothered to convert them until Pope Gregory sent missionaries. The Goths were likely converted by a combination of the Arian missionary, Ulfilas, and Emperor Valens. The other Germanic tribes were converted by the Goths. The Goths showed a markedly greater missionary fervour in the 5th century than the Roman Catholics and successfully sent missionaries to a number of pagan Germanic tribes to convert them to Arian Christianity. For instance in the 5th century, the Visigothic king, Theodoric II, successfully sent the missionary Ajax to convert the Suevi to Arianism.

On the other hand, Roman Catholics were rarely interested in converting non-Roman citizens to their Christianity, especially those living outside of the empire. Although the Goths heretical tendencies were regarded with some hostility, it was also regarded with some relief by the Romans as it differentiated them religiously from the Barbarians and helped to draw a divide. The exception being the Franks who converted to Catholic Christianity to gain vital support from the Gauls against the Arian Goths after the western Roman Empire had already fallen.So Roman Catholics for most of the 4th and 5th century were largely uninterested in converting non-Romans. It was only in the 6th century that Catholic missionary activity picked up, which is why you have Pope Gregory sending missionaries to the Anglo-Saxons.

The 8th century Anglo-Saxon writer Bede records that the ‘Britons’ (that is the Welsh) always remained Christian, but speaks contemptuously of them because of their past refusal to convert Anglo-Saxons to Christianity. Bede writes:

“Among the other unspeakable crimes, recorded with sorrow by their own historian Gildas, they added this – that they never preached the Faith to the Saxons or Angles who dwelt with them in Britain. But God in his goodness did not utterly abandon the people whom he had chosen; for he remembered them, and sent this nation more worthy preachers of truth to bring them to the Faith.”

Welsh bishops were even asked by one of Pope Gregory’s missionaries named Augustine to help them convert the Anglo-Saxons to Christianity, but they refused.

Therefore, Christianity was certainly widespread in Britain by the 4th and 5th century and the Britons remained Christian (the unconquered western parts that is now known as well Wales at least) throughout Anglo-Saxon domination of Britain. The Anglo-Saxons likely remained pagan for almost two centuries due to a lack of Catholic missionary zeal and the fact that the still Christian Welsh (the Christians closest to them) were extremely hostile to Anglo-Saxons and had no desire to convert them to Christianity as pointed out by Bede.

Sources:

The Cambridge Medieval History Series volumes 1-5, J.B. Bury

Orosius, Histories

Gildas

Bede, The Ecclesiastical History of the English People