I disagree. Its existence must be common knowledge, but in places it is not unrealistic to expect that members of a certain sect might want to practice secretly to avoid persecution.
This is partly why my suggestion is not to create whole new parallel religion system that is completely secret, but to add a couple features to the existing one. Other religions would still see that a religion exists and be able to declare it to be evil. Priests would still not be able to hide their affiliation. And to have meaningful followings that give you access to the truly powerful religious options, you will of course have to expand temples past the point where they can be kept secret and preach openly to the populace.
I simply think that the option to keep *some* things secret in certain places or under certain circumstances would be overall a positive enhancement to the religion system and allow persecuted religious minorities to attempt to hide from the authorities in realms where they are not welcome. Think of the intrigue!
Of course Tom's opinion is the last word, but I hope that these concepts will at least be given some thought.
