Over on Dragonsfoot, I responded to a request for ideas on how to limit magic in a low-magic campaign and had some positive response to my solution for limiting clerics, so I'm posting it here, with expanded material.
By default, clerics are supposed to serve their patron deities, but it's up to the GM how to handle this. My suggestion: track a Piety score, openly on the player character's sheet or secretly, tracked by the GM. Assuming the old-school approach to clerics (no spells at first level, spell levels range from 1 to 5,) Piety starts at 0 and can reach a maximum of 5.
Every time the cleric does something that serves the deity's will, make a reaction roll for the deity, adjusted as feels appropriate. On a good result, Piety increases by 1. At the GM's option, an excellent reaction may mean an increase of 2. Conversely, make a reaction roll for affronts to the deity or violation of taboos, to see if Piety decreases.
How does Piety affect clerics? Option 1 is to limit the maximum spell level usable. Piety 0 means the cleric doesn't have enough faith to cast any spells. Piety 5 means the cleric can cast any spell available, based on the character's level.
Option 2 is to limit the type of spell usable. A simple interpretation is to allow healing spells at Piety 1, purely defensive spells at Piety 2, and all other clerical spells at Piety 3+. A more elaborate approach is to breaks spells into more spell types or create unique spell groups for each religion, perhaps with a domain approach as used in AD&D 2e: you get access to your deity's primary domain at Piety 1, secondary domain at Piety 2, defensive spells of any domain at Piety 3, and other spells at Piety 4.
I have some more thoughts on Piety for later posts.