I've never liked this whole "what if religions want to be secret cults" thing. If you want a secret cult, do a secret society. A religion requires at least one sizable temple. This is not some little hole in the wall people can scurry to without notice. This is a large building with staff. It costs more than some recruitment centers. And most religions have enough infrastructure in temples and shrines to make them equivalent in total investment to a duchy, and quite a number have as much or more than realms.
I think the religion game should be more public, people should be standing up and having their beliefs known, and religious titles should be given equal weight to secular ones. The highest of high nobles of a realm joining a religion is not something that could be secret, nor should it be.
