One of the things I really like about Alpennia's ~magic system is that it's historically coherent with Western history, where the Catholic church held total supremacy over much of Europe for over a milennium, BUT, you can definitely see the cracks that mean an alternate interpretation would totally blow the whole system wide open.
One of the things about being an ex-Catholic history nerd is, it's hard for me to really invest in "our world, except magic" AUs where magic is this totally separate branch of institutional power in Western society, where there is the monarchy/government, there is the Church, and then there is The World of Magic—because the Church, being the largest branch, has always tried to eat its lesser kin; there has been so much warfare about whether the Roman Catholic Church holds precedence over secular governments, or whether secular governments can tell the Church where to GTFO. So it is very fundamentally implausible that, if magic truly exists, the Church has not annexed it and claimed to be its only true source.
Daughter of Mystery is about the "mysteries" of the saints, where "mystery" means both "a secret ritual" (like the Elusinian mysteries) and "an act of religious ministry" (like medieval "mystery" plays). It's an adaptation of things that exist in our world, where Catholic veneration of the saints has always included asking different saints for different things, and honouring them with specific actions, prayers, pilgrimages, and items. That means everything from walking hundreds of miles with bare feet to worship at a shrine, to sighing and chanting, "Anthony, Anthony, turn around, something's lost that must be found" while spinning in a circle, before continuing to look for your car keys.
In this world, the mysteries work. Not all the time, not for everybody, but just often enough to be miraculous. And the Church traditionally holds that God and the Saints are just very choosy about when they bestow those miracles, and it's the job of ordinary people to pray humbly and dutifully and bow their heads when the blessings fall down.
But slowly over the centuries, theologians, philosophers, and eventually, thaumaturgists, have been observing that there is a pattern to when and how the mysteries generate miracles. There are some people who are especially sensitive to seeing or hearing the working of miracles, who can say whether a prayer was answered or a mystery was successful; and there are some people who, much more reliably than others, have their prayers answered. If they work together, they can use trial and error to create far more effective mysteries that produce more reliable results.
This has led to the creation of "mystery guilds", because the guild was the medieval framework for a closed group of people who meet to work together for a specific cause; in this case they were not tradesmen regulating a profession or philanthropists raising money for a cause, but people working to celebrate mysteries in hopes of creating a desired miracle. (I don't know if it comes up in later books, but I imagine the Freemasons in this world are buck wild.)
The Church officially condemns what is called "the mechanistic heresy", which says that miracles don't come from God, but are the sum of the people involved and the thoughts and words and actions they perform. Thaumaturgists have to step very carefully around this issue, because it is actually pretty clear that either case is equally possible, but they want to not die at the hands of the Inquisition. So there's this very real tension where thaumaturgy that was scandalous and near-heretical in its day (which I'm guessing to be the 12th century?) is now essential to understanding modern philosophy, so a nun who disapproves of the field as a whole nonetheless has to teach parts of it if she wants her student to understand why Christian thought and worship of the early 1800s is the way it is—but she won't give that student access to the entire book, just carefully chosen excerpts. And even if that book can be obtained from other libraries, other books of thaumaturgy are so at risk of being deemed heretical that reputable printers won't print it, and reputable booksellers won't sell it.
It reminds me a lot of the practice of human dissection in European history. During the Early Middle Ages, the Church discouraged the practice, which had been more common in Ancient Rome; it held that human bodies were sacrosanct, and could not be in any way damaged or altered after death, because when humans will be given our "new flesh" on the day of the Resurrection, it will be our actual physical bodies that will be revived—so your body had better be in a revivable state, and not cremated or taken apart or anything else.
But slowly, in the 11th and 12th centuries, it became clearer and clearer (thanks in part to physicians from Jewish and Islamic traditions) that dissection was a necessary part of medical science, because doctors and surgeons will just be fundamentally worse at their jobs if they don't have the kind of detailed knowledge of anatomy that only human dissection can provide. So in fits and starts, various regions and bishops and popes made the procedure more legal, even as they limited who could be dissected to those least likely to be deemed fit for the eternal hereafter.
Which, like... it is completely off the chain for us to live in a world where autopsies are performed on everyone, good citizen and hardened criminal alike, in the case of suspicious death; where it is routine to take the vital organs of a dead person out of their body and transplant them into a new one. We couldn't imagine law enforcement without forensic pathology, and we're comfortable with medical science experimenting with the human body in a way that treats nothing as sacrosanct, except knowledge and truth.
So Alpennia is at the jumping-off place for this brave new world. They've been a solidly Catholic country, wedged between France and Switzerland, only slightly touched by the wilder excesses of the Enlightenment and the French Revolution; Napoleon's wars ravaged them somewhat, but a new generation that barely remembers them is coming of age, with the knowledge that all the old truths can be thrown down—if you throw hard enough.
These books are so deeply nerdy. So very much. I love them so.