I think the default beliefs in BNW are also too weak; many of the mod's beliefs are still an improvement.
That may be the case. I'm not really sure which beliefs seem uninteresting to me because they're under-tuned, and which ones just don't have interesting/relevant effects for my desired win condition, civilization, or play style.
I can look into some kind of reference for belief changes. We haven't updated the wiki page in a while, mostly because there were changes galore in progress and it wasn't clear yet what was going to work and what wasn't. That was then. I'll start looking over things more. The big one though was shifting around some as follower/founder/reformation beliefs. As I recall, this was not popular at the time for some of the reasons that are outlined.
Other than the missionary cost, most of this actually stems from changes to the piety tree anyway from the sound of it. I think the missionary cost increase was to decrease missionary spam by the AI, but I had thought there were limits on how many they could have at once (which means instead they end up with lots of great prophets for conversion, particularly with the current piety-reformation setup).
I think the next game I play I'll screenshot the belief screens then put them in text later. I'll add that here, or somewhere for documentation purposes and to better facilitate discussions.
I'm actually more unhappy about the beliefs than the piety tree, though piety isn't blameless.
Is AI missionary spam really something that we're concerned about? Of course I wasn't around for that discussion, but why is that a bad thing? And how many missionaries could they possibly squeeze out, and is there any reason that the AI can spawn them faster than a human?
A couple of things about the different ways religion spreads to keep in mind is spies "can" have an input in pushing the faith on the city they are in.
Also trade routes also can have an influence in spreading the religion to those cities you trade with. Conversely they have a limiting factor on opponents' religions if you both are trading with the same city.
I've never heard of spies interacting with the faith system. Even for Arabia, the trade system is a weak way to spread religion. In my last game, Spain and I (Arabia) were fighting over the hearts and minds of the Japanese, whom were situated in between us. Isabella had the "+25%/50% religious pressure (with printing press)" belief and was city spamming and my trade routes absolutely could not compete.
True, sometimes I was out-pressuring her, but by so little that it would take an entire age to flip the city.
I think we need
all the religion spreading beliefs in the reformation belief pool. That includes +pressure, +pressure range, and all beliefs which buff prophets and missionaries. This way when you've invested 5 points in piety, you're declaring to the world "I mean to be a leader in religion." This will give incentive to finish the policy tree instead of cherry picking, and comes with the benefit of being able to out-religion more casually religious civs and be competitive with the more serious ones.