I'm happy to provide more detail on a proposed mystery cult. I just didn't want to be presumptuous.
So here's my proposal. It's just a proposal, mind. I'm not assuming that anything is set in stone or even that you'll want to go with my ideas. But here they are.
My concept is an organization devoted to the study and advancement of scientific learning, particularly the alchemical arts. The Society of Alchemists claims to be built on the foundation of a Mystery Cult far older than the Order. It asserts that its predecessor organization had its origin before the birth of Christ, and was privy to all the secrets of the lost Library of Alexandria, which included a secret trove of books related to magic and alchemy that the public never saw. And while much of this knowledge was lost when the library burned, the old cult was said to have saved many of these secret books.
Alas, with the fall of the Roman Empire, the original Mystery Cult was lost, and its secrets with it. When the new Society of Alchemists was founded centuries later, it had only the tiniest scraps of the vast knowledge once held by its ancient predecessor. Chief among its goals was to recapture what was lost, both in the secret arts of alchemy and magic as well as in many other areas of learning. In the time since its founding, the new Society of Alchemists has managed to piece together some of what was lost. But always there is more to learn. They have stories of what the old cult was supposedly able to achieve using magic and alchemy, and hope to someday equal or even surpass that knowledge. Ultimately, the cult hopes to perfect the mind through study and the body through alchemy and magic, allowing for its members to live a perfect existence of eternal scholarship. At present, however, that's just a dream.
Its current members are generally scholars, librarians, or other keepers of knowledge, and in their public lives they often work with learning, teaching, or the creation or preservation of knowledge, often related to science, medicine, and mathematics. Each generally has a subject that serves as a focus for his learning. But in secret they seek to recapture the mystic learning of the ancient cult, the secrets of alchemy once known to the Greeks and Romans, but now lost to the modern world. They are said to have a secret library somewhere in Mythic Europe where they keep copies of rare texts on magic and alchemy, and are always eager for more.
So, the general idea is a group seeking to recapture the lost scientific knowledge of the ancient world, and expand upon that. The alchemy is learned both as a means to perfect the human body so that they can devote an eternity to study, and as one of the more esoteric areas of learning, and so worthy of study on its own.
I was thinking that after his gauntlet, Galan joined the covenant of Schola Pythagoris in Stonehenge so he could study natural philosophy with Astrolabe of Jerbiton. During this time he showed a devotion to learning (I was planning on having him spend xp to up his Philosophiae) that made a member of the Society (Astrolabe or someone else?) think he might be a good recruit. Over time they slowly sounded Galan out until they finally approached him with a veiled invitation to join.
I don't know if any of that is at all appealing, but that was the concept I had in mind for a mystery cult. It seemed on par with the sort of mystery cults described in TMRE. Ill be interested to hear your thoughts.