It is reasonable to assume that some people unfamiliar with Christianity/Islam will assume that they actually ARE Mystery Cults, and that beyond the publically available myths and teachings, there is one or more "true mysteries" only shared with those who graduate to the "inner circle".
In-world Wild Mass Guessing, except it won't sound extremely tinfoil hatty to someone whose frame of reference is that religion=mystery cult.
It is reasonable to assume that some people unfamiliar with Christianity/Islam will assume that they actually ARE Mystery Cults, and that beyond the publically available myths and teachings, there is one or more "true mysteries" only shared with those who graduate to the "inner circle".
In-world Wild Mass Guessing, except it won't sound extremely tinfoil hatty to someone whose frame of reference is that religion=mystery cult.