Christianity plays a huge part in the traditional Arthurian legend, which high Middle Age writers magically teleported into their own time. But when you move the story back into the background where it would have taken place if it is has historical roots, late 5th and early 6th centuries, the religious picture becomes much murkier. Particularly since we have very limited sources (written, archeological, etc) that contribute to our knowledge of the period.
When developing your own legend, it’s always good to start with what you know. Fiction isn’t bound by any of this, of course. But if it gets too far afield it becomes Alternative History or Fantasy. None of that is really my intent.
THE HISTORICAL FACTS AS WE KNOW THEM
Prior to the Romans, what is now the UK and France was inhabited by Celtic people. Their religion is believed to have included strong elements of paganism, naturalism, and ancestor worship. Their priests were known as druids (maybe). Were these the same beliefs that led to the construction of the great henges such as Stonehenge? Nobody really knows.
Following the Roman Empire’s conquest of Gaul (58–51 BCE) and southern Britannia (43 CE), Celtic religious practices began to display elements of Romanization. The Romans during this period were not big religious converters, however, and in general people were left to practice their own beliefs (as long as they didn’t foment revolt).
For some unrecorded reason, the Romans began a systematic purge of druids in the 1st and 2nd centuries AD.
Following the conversion of the emperor Constantine to Christianity, the new religion began to spread through Roman lands. The Edict of Milan in 313AD granted people of the empire the right of free and open observance of their own religions. It was specifically intended to stop persecution of the Christians, but the wording of the edit did not specify any favored religion.
In 380AD, Christianity became the official religion of the Roman empire. Complicating this picture is that many of the Roman soldiers practiced Mithraism, a much older Persian religion (which was more popular among soldiers than the populace at large).
30 years later, the Romans withdrew their legions from Britain and left the inhabitants to fend for themselves. After the Romans withdrew, pagan Germanic tribes rapidly overran Southern England *** except, of course, for that brief shining period when King Arthur held them at bay. The invasions destroyed most remnants of Roman civilization in the areas held by the Saxons and related tribes. These included economic and religious structures, although there is some evidence that traces of Christianity survived.
Augustine of Canterbury led a mission to the court of King Æthelberht around 597, which eventually led to the conversion of the Saxon kings in the early 7th century.
OTHER LITERARY TREATMENTS
The Mists of Avalon frames the conflict between Arthur and Morgan le Fay as between the old religion—paganism—and the new religion—Christianity. This is radically different than the traditional Arthurian legends, but is not unique. In particular, Avalon is often considered the center of what is left of Celtic paganism during the times of King Arthur.
RUSTY’S WORLD OF KING ARTHUR
In the story of Sir Kay, Christianity is brand new to Britain. Merlin “dies” in 504; a year later, the first official Christian mission to the Court of Arthur arrives (the implication is that Merlin would not have allowed such a thing before). Prior to 504, there was little knowledge of Christianity in the court.
How possible this would have been depends, basically, on how “Roman” Arthur’s realm was. Jack Whyte has an entire series of novels where Arthur is basically defending Roman civilization against the barbarians (curiously, in those novels, Arthur is Mithran).
I have elected to cast the realm as containing the last remnants of old Celtic culture instead. It makes the story much more fun, in my opinion. Not to mention, departures from existing traditions is what we Arthurian novelists live for.