Religion plays a very big part in Europe during the Dark Ages. There is no such thing as a non-religious knight in the sense that there is at least lip service paid to religion by everyone. There is, however, a lot of variance in piety and strength of religious conviction; where a player knight falls on that spectrum is up to the individual player.
There are numerous religions the players may encounter. These will be listed below. The Big Three (British and Roman Christianity, British Paganism) which will be available to PKs in the 485 starting year will be listed first, followed by a few of the more commonly encountered continental religions. Of interest to the reader will be the lack of inherent conflict between Christianity and Paganism. The default attitude is one of acceptance tinged with a slight sadness as the Old Ways give way to the New. In actual play it may be that religious tension is something the group wishes to play up which is entirely viable.
It should be noted that the distinctions below are not really in the Arthurian myths. In the Vulgate Cycle on all of the knights are squarely Medieval Catholic with a few distinctly British quirks. British Christianity, as described in KAP, is not present. As well, knights were never Pagan; only Morgan Le Fay, Merlin and the Ladies of the Lake were described as Pagan of the fleshed out characters. British Christianity and British Paganism being available to PKs are conceits of the game and no surprise should be felt if a player reads the literature only to find uniformity of religious beliefs amongst the members of the Round Table.
This is Catholicism, ca the time of Augustine. It believes in Papal supremacy (and that really does mean infallable supremacy at this time) and gives local leadership to Catholic bishops. In Logres for the duration of the campaign this is a very city based, sophisticated religion. The bishops hold little sway outside the major population centers. Again, the default is that there is little tension between Roman Christianity and the other local religions, though they tend to be a bit more wary of Paganism; British Christianity is tolerated just fine. In terms of mood, think of an emphasis on ritual and urban values.
Not terribly different from Roman Christianity with a few deviations. First and most important, power is held by monastic organizations. Monks reign supreme in British Christianity with small local parish priests making up the difference. What heirarchy there is is extremely decentralized; there is a nominal head of the British church who is invariably some weird hermit living in a Welsh cave somewhere. Perhaps strangely, the Britains claim to be older than the Roman church, tracing their lineage to Joseph of Arimathea’s arrival in Glastonbury very shortly after Christ’s death; this is also, incidentally, why Grail imagery and lore play such a large role in the religion. In play, this is the religion of most of the British Isles and has a distinct rustic feel to it.
The most important thing to know about British Paganism is that we don’t know anything about British Paganism. Not really. The fact is that the ancient Welsh/Celtic culture of Britain did not write things down and, for all the talk by modern Britains of “Britains will never be slaves”, they were invaded dozens of times prior to the final 1066 purging of their indigineous culture. It’s lost. This gives the player a lot of leeway in defining his PK’s beliefs. Generally speaking, the Salisbury campaign will default to a combination of the Mabinogen and general nebulous Earth worship. That said, codifying British Paganism and fleshing it out may be something the group is interested in doing through play in a collaborative fashion.
The religion of the Saxons, both from the continent and on the Isles, is essentially the Norse mythology we’re all familiar with. It places a premium on physical valor, blood sacrifice and stoicism in the face of hardship. It’s a hard, savage religion that most British have a hard time with.
Arian Christianity is extremely widespread on the continent and was, historically, the biggest rival to what became mainstream Christianity. It was so widespread, in fact, that most of the pre-Nicean Creed documents by the early Church Fathers were written specifically to counter Arian doctrine. What was the big to do that got Arianism labeled heresy and stamped out? Arians claimed that Christ was a creature, that is He was created by God the Father and was, therefore, less divine and subservient to God. That’s it. Pretty minor distinction to today’s ears, where nobody ever meditates on the paradox of the Trinity, but a huge deal in Late Antiquity.
Presented as pretty much the same as today. Does not recognize the primacy of the Pope and views the Patriarchs (Rome, Alexandria, Antioch, Constantinople, Jerusalem) as more or less coequal, though Constantinople is given a bit more deference than the others. Notable differences with Roman Christianity are no belief in original sin or predestination, an emphasis on meditation, a different calendar and a literary tradition (at the time of the game) of solitary hermits battling Satan in strange ways.