[Skarg]

The converse though is that if the GM is using a published system, or lets everyone know what the powers of every religion are, then everyone knows, even people who shouldn't.

Of course GMs can change it, but many won't. If there are published religious powers, I'd suggest publishing a variety of examples of types.

In practice, what I do is reveal the information to players gradually.

If it's an interest that develops during play, the gathering of information and the choice to commit to study or not can be played out appropriately, which I think is nicely interesting and logical. It's happened in other areas as well, when PCs join guilds or other groups and then may get disappointed when they find out what's really offered and/or expected of them.

For character generation, that means we ask each other questions to determine if there's a match between their interests and what the religion is actually like. I start with commonly-available information about the religion, and ask them what sounds interesting or not about that, and they can ask for more details and so on. If there are some secrets that would best not be let out, I'll carefully not provide a solid answer, possibly even if they decide they do want to join.