King Arthur and His Syncretistic Knights
Setting aside mysteries for the time being, DOB and I started reading Rosemary Sutcliff's retelling of the Arthurian Legends, The Sword and the Circle. So far I'm impressed with the version (but I have always enjoyed Sutcliff): it has the right tone for an ancient legend while remaining entirely readable, and it handles the immorality and occultism at the heart of the legend well, neither glamourizing it, camouflaging it, nor going into excessive detail. Alas, unlike the gorgeous illustrations for her retelling of the Iliad and Odyssey, there is only a cover illustration and it looks like three modern teenagers in Halloween costumes.
When you look at the whole story, the mix of beliefs is indeed strange. People go to the church feasts, yet they still look for omens from the stars. Children are raised in convents, but also study black magic. Incest is taboo, but adultery is acceptable--and so is revenge killing, or even killing for the fun of it, as long as it's a fair fight. It is not yet a Christian world, but a syncretism of Christianity and the old practices. There is a God, but He seems too far away for practical purposes. It reminds me of what I've read of some areas in West Africa today, where people may profess Christianity or Islam, but still do voodoo on the side.
Because the pagan beliefs are still strong, there are a lot of random or unknown quantities in the stories. Sitting at the end of centuries in which the universe has been accepted as orderly, first as created by God and then by the laws of science, it is harder for modern inhabitants of Western civilization to appreciate the world of Merlin, where things just happen, and don't have to make sense. But it has been a common outlook on the world, and is likely to become more so, as a society that no longer has an intellectual faith in God begins to lose its faith in science.
This is not the world of the Middle Ages, whose romantic lens we usually see it through, but the Dark Ages, in which semi-barbaric peoples struggle to survive after civilization has disintegrated. People commonly muddle the two, and then blame the resulting mess in their minds on the ascendancy of the church. But the Dark Ages were so dark precisely because the church had so little influence. It was in the Middle Ages, when the church became the final arbiter of morals and manners, that civilization was able to stabilize enough to move forward after the wreck of Rome.