If my players are arranging to stage dramatic conversions that add to the game, they're welcome to the XP!
That's the trick through isn't it, and why a DM has to police the issue to begin with. It's one thing for a couple players to talk in and out of game/character about possibly doing something amazing, from changing their faith to building homes for the homeless. When this is true and honest RP, it's great!
But sometimes it isn't good and honest. Sometimes it's "Hey Bob I want some XP, let my character convert yours to your religion!" "Well okay Phil, but how will that get you XP?" "Well Bob I told the DM that my character's goal was to convert everyone, so if you just write my god's name on your sheet, i'll get XP!"
Personally, when a player asks "What can I do to get more XP?" it says a one of two things to me.
A: That player is greedy and wants more for themselves.
B: The game lacks enough XP/opportunities for the players.
A player asking how they can act on the desires, goals, and wishes of their character is a great thing. Working that out with other players and the DM is also a great thing. But "asking for XP" strikes me more as a player trying to get something they want without having to actually earn it.