Cosmologically, I can see them working in two ways: the divinity is "real", enough, but the cult misrepresents or manipulates the worship in such a way as to benefit itself, (e.g. Gark); and secondly, the object worshipped is itself "fake". It's possible for this to occur do to weirdness like Short Worlds, where you have what's really a closed magical loop: you can "create" an otherside domain which isn't a permanent, necessary part of Glorantha, but simply reflects or acts as a "repository" for the magical energy and patterning that's placed in and on it by its worshippers.
It's hard to say whether any given religion _really is_ a short world based 'scam' (or inadvertant trap) -- by their very nature they would tend to _look_ exactly like a real religion, god, and otherside, unless you do the accounting that tells you it's a zero-sum game. (Or unless you find that the short world has gone "poof" one day, and the chief priest is legging it to Teshnos with the magical proceeds.) It's likewise possible that some cults may have elements that are both "real" (proceeding from the underlying transcendent reality of the Gloranthan cosmos), and that are "short" (magical creations of many generations of worshippers).
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