Additionally, even if the only effect of this was to have characters join a religion because it would cause them problems if they didn't then that in itself would be an achievement given the number of characters which currently remain pagans.
I'm not convinced it would make the religious game, let alone the game overall, any better. I see as much potential for negative impact as I do for positive. "More people will be in a religion" doesn't sound like a great selling point, when in addition you will be adding a lot of aggravation on the part of quite a few players who want nothing at all to do with that part of the game.
Plus, given the way religions spread, something like this would give realms a solid incentive to try and ensure they have one religion in their land instead of multiple coexisting ones
Or perhaps it will cause realms to purposely mix more religions, in order to prevent any one religion from gaining power. I already know of characters who purposely take active measures to prevent any single religion from having too many followers. Keep the followers down, and remove all potential negative influences.
I really don't see this as the universal panacea that you think it would be.
Combined with "the locals are disgraced that their local lord is indebted to the Church" could give a sure boost in religion finances.
That could be an interesting option. But I don't know that it would really drive up church finances. More likely an uprising against high membership fees, resentment against the religion, and loss of members.
IMNSHO, forcing people to play the religion game is a losing proposition. People that want to play it, already do.