That's being addressed somewhat, mostly in the second chapter, which deals with the history and hierarchy of the organization itself. The Sisters have, throughout history, clashed with the major monotheistic religions which have been, historically speaking, rather intolerant of paganism. Before the rise of Christianity, the Sisterhood, mostly based out of Ancient Greece and Rome, was powerful both magically and politically, but the conversion of the Roman Empire to Christianity began their downfall. They struggled to survive under Christian and Muslim rule for centuries until the Industrial Revolution, secularized Western society a great deal, allowing for their eventual return to the mainstream, so to speak. The late 20th century brought about a much greater sense of true religious freedom in the West, allowing the Sisters to operate, for the most part, in the open for the first time since antiquity.
It's worth noting that I'm writing from a POV that, unlike fantasy worlds where the existence of the gods is a known factor, divine magic in a modern arcana world is based on mass faith, meaning the more people believe, the more powerful the magic, which is why the Sisters struggle to maintain their religious identity. The great monotheistic religions, like Christianity, don't believe in "magic" and, those, have very few clerics capable of using divine magic (i.e., "miracles"). Those that do, are usually considered great prophets and saints. The irony of it is that, likely, ANY cleric of any religion could likely use divine magic if they actually, truly believed they could, but the doctrine of their faiths usually prevents them from even trying. This is why the Sisters are often branded as "witches," because their faith allows for the use of divine magic, which most of the world believes to be inherently "evil." Even in modern days, the Sisters are not free from persecution. Of course, if they also happen to live in a world where arcane magic is possible and common, their magical abilities are likely less of a hindrance to them. How that all plays out is honestly left up to the GM.