I agree with you that most magi won't be in a Mystery Cult. But the Cult of Mercury isn't a Mystery Cult. It's a Hermetic Societas with open membership. Maybe you thought I was talking about the Neo-Mercurians, who are an off-shoot Mystery Cult developed from it? As a Hermetic Societas, it's more like the more dominant of the two dominant religions (Christianity being the other) in the Order.
Essentially, Mercury Cultists are Mediterranean nature-worshippers, and their ceremonies focus on anthropomorphizing forces like weather, death, love, war and so on.
they welcome any magi who wish to join their group.
The reason I said "more dominant" is that they're also heavily integrated into the general Hermetic society. Their leaders are like the religious leaders who bless non-religious events in our world.
At Tribunal, the flamines often perform an invocation or a blessing before the event, and fulfill other duties such as leading Hermetic naming ceremonies, witnessing oaths, or conducting funereal rites.
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As for the numbers, I was exaggerating. It need not be that big, of course. But canonically it really is huge and could be that big. We have statements that the Cult of Heroes is one of multiple smaller sub/side-groups of it. The Cult of Heroes alone is mostly House Mercere, with a few Gifted members and a bunch of unGifted members. And then there are more Mercere outside of the Cult of Heroes, with about half the Gifted Mercere and more Redcaps who aren't in the Cult of Heroes. And despite all these Mercere
[House Flambeau] are still most represented. Through Priamitus they have always had very close ties to House Mercere, and they welcome any magi who wish to join their group.
The Cult of Mercury includes magi from many different Houses
Additionally, we know those in the Cult of Heroes and the leadership of the Cult of Mercury are pagan, yet
A good many cult members consider themselves Christian as well as Mercurian.
So the books aren't very precise. But putting them all together, it's essentially impossible for there to be fewer than 50 or so magi (including unGifted Redcaps). The lower limit is probably closer to 75 or so. Even 75 requires straining textual statement. Having worked with stats a lot and putting all these statements together, I have a hard time believing the implication is under a hundred. Yes, that is well under a quarter of the Order, but it really is huge, and the text doesn't rule out it being a quarter or more.
Now, that is just canonical stuff, and need not exist in game. I was just going off it being a major, open Hermetic Societas that is essentially one of two religions within the Order to which many belong.