While many fictional accounts of the Dark Ages describe conflict between pagan religions and Christianity, that seems to be a product of the medieval mind, rather than an accurate analysis of Dark Age religion. For there to be conflict there must be a power relationship as well as organization, and for both the pagans and the Christians in Wales in 655 AD, there were neither.
When the Romans conquered Wales in 43 AD, although Rome was not Christian at the time (Emperor Constantine didn’t convert until 311 AD), the legions systematically wiped out the reigning religion of Wales at the time, which was druidism. Why did they do this? The Romans themselves were pagans, with a pantheon of gods and goddesses. Why did they not simply incorporate the native gods into their own religion as they did in most other places, and as the Catholic Church did later throughout the world?
The difference was that the druids formed the basis of a nationalist movement in Britain—and throughout the Celtic world. To quell it, the Romans systematically destroyed the sacred sites and groves, particularly on the island of Anglesey, prompting Boudicca’s revolt in 61 AD. The Romans defeated her, and the end of the revolt spelled the end of organized druidism in Britain.
Thus, in the time between this momentous defeat and when the Roman empire became Christian, there was a lengthy vacuum, both in religious leadership and belief. Christianity came to Britain in the first century, not long after the death of Christ, but was no more organized than paganism without the druids. Wales was far from Rome and the seats of learning, and when the Roman legions left, the Christian religion was cut off from its roots.
Christianity in the Dark Ages, then, was one of several available options in Wales. By the mid-600s, Christianity was growing more organized, but it was a religion based around monasteries. There were cells of monks and hermitages, but few, if any, churches as we understand them. There were also strong pulls towards different sects within Christianity, and strong resistance to the Roman Church, with which the Welsh Church did not reconcile until 763 AD.
Even up until the death of Llywelyn ap Gruffydd in 1282 AD, the Welsh Christians were unhappy with conformity to Rome, especially as the Church kept excommunicating their Princes for not bowing to England. Welsh laws did not conform to the Church’s teachings well into the Middle Ages. Most notably, women in Wales had a higher status compared to the rest of Europe, divorce was easier, illegitimate children could inherit, lords levied fines instead of executing criminals as punishment for crimes, and the punitive forest laws of English/French feudalism were absent.
Religion in the Dark Ages was at the intersection of superstition and mythology. The old Welsh gods had not been vanquished, but were everyday participants in daily life. They were random and capricious, just like the weather. Jesus Christ, if adhered to at all, brought a message of personal salvation and belief in heaven, rather than the Underworld. Christ allowed a believer control over his ultimate destiny.
Eventually, it was Christianity that incorporated the pagan Welsh gods into its pantheon of saints, accommodating the old beliefs. In the Spoils of Annwn by Taliesin, a Christian, the final two stanzas of the poem rail against dissolute monks, comparing them to wolves or wild dogs and end with a prayer to the Lord and Christ. At the same time, the bulk of the poem describes Arthur’s descent to the Underworld and his battles in the world of the sidhe. This blend of pagan and Christian is the hallmark of Dark Age Wales.