Ryan posted a comment on a A Crisis of PC Faith Answered that brings up a important aspect of how I run my campaigns.
This will be a little more difficult for this particular cleric. In the area of the campaign world where the characters are operating, there is only one legally sanctioned religion, and the character is a cleric of a different religion. As it stands, all non-sanctioned religions are forbidden from building temples or having more than six priests in any given town. His religion does have shrines in rural areas, as the teachings are becoming more popular with peasants and common folk, so perhaps he can develop a network of patrons in rural areas or something to that effect.
In some ways what Ryan describes is an ideal situation for a PC. In addition to the cleric character growing in game terms, the small size of his faith means he can have outsized impact on it's subsequent history.
If the setting will be used in later campaigns. Keep notes on what this person does (as well as your other players).
Then weave it into the background of the next campaign that is run using this setting.
This will almost guarantee player loyalty and add an amount of coolness to your setting.
Plus it has an added benefit of you not having to explain a detailed aspect of your setting's background to existing players. Just remember to keep a cliff's notes version to pass out for the new folks.
The main downside is players using knowledge their character doesn't have. I don't get rigid about this. After all it is tough at time to get players to play attention to the setting at times. However I do make it clear that abusing out of game knowledge is not a good way to earn experience in my games.
In a world where we compete against dozens of other games for time and attention; the ability of tabletop RPGs to dynamically change to reflect what a PC does remains a singular advantage. Use it!