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.
If you are practicing as a
noble secret sect, that works perfectly as a secret society. If you want to gain common followers, you are going to have to make it public sooner or later.
For temples and shrines to be of any use, they must be known about. For a sermon to be able to convert people, you necessarily have to bring in people not already part of the faith.
If you just want to keep your membership in a religion secret from your realm, then don't become a priest and don't accept a lordship. Then no one who's not already in the religion can identify you.
In the end, you can choose: secrecy, or power over the commoners. That's the tradeoff you have to make.
