For reasons I won't discuss, in an article about rules for how to handle minor divine intervention in a campaign, my rules were radically changed from what I wrote. Personally I think my version works better and is more uniformly fair, so I'm posting it.
This system is designed to be equally fair to dirty peasants and to glorious adventurers (at least in terms of bang for the buck), be cheap enough to allow commoners to benefit from it, be limited to very small effects (so no heal spells arising from small donations and random rolls), and have a reasonable cost relative to actually purchasing a spell of the same type as the intervention.
Many deities are casually worshipped by the people
of the world just by making donations to a temple or shrine of that deity.
If the character has recently (within the past week) donated to the church
of a deity, and he is in a situation relevant to the portfolio of that
deity, the deity may intervene in some small way to aid that character.
When the circumstances are appropriate, the player should point out the
situation to the DM, and if the DM agrees, an intervention roll is made.
The chance of a minor intervention is equal to 1% x the number of silver pieces donated by the character to that faith in the past week. If the roll is a success, the character immediately benefits from a guidance, resistance, or virtue orison (which may reverse a just-failed check or saving throw). If the roll fails, the character receives no intervention and receives no other chances for intervention from that deity until another donation is made.
Should a group of people donate make a donation as a group, any one of the group can call for an intervention based on the total donation; failure means that no other rolls for intervention based on that donation can be made.
One could argue that the gods have no interest in money. Not true. Their temples need maintenance (and new temples need to be built), guards need to be paid, priests need to be fed and clothed, magic items need to be paid for, and so on. Gods themselves may need no money, but to accomplish their goals in the world it helps to have gold.
Money is not the only sort of appropriate donation. Goods (including food for the priests or to be given to the needy), items of significance to the church or temple (such as remains of a long-dead hero, or a religious relic, or even church-created magic items), and services (from repairing an old church's window to digging a garden for a monastery) are all appropriate donations, and should have their value converted to silvers for the purpose of computing the intervention chance (based on a common laborer's daily wage of 1 silver piece or other hireling wages from page 149 of the DMG).
Example: Narv steps between an angry owlbear and his unconscious ally Phip, making sure that beast doesnít carry away his fallen friend for a meal. The owlbear attacks Narv and reduces him to 0 hit points. Narv's player reminds the DM that Narv donated 5 gp to the temple of Zelm (the god of protectors) yesterday, and asks for an intervention roll, as he is acting as a protector for his fallen friend. The DM agrees that this is an appropriate circumstance for an intervention by the Zelm, rolls percentile dice, and gets a 30. The god intervenes by targeting Narv with a virtue spell, bringing him to 1 hp. Narv is able to attack the owlbear and kill it on his turn. Without the intervention of the virtue spell, Narv would have been at 0 hit points (disabled) when he attacked the owlbear and would have dropped to ?1 hit points for performing a strenuous action while disabled, putting him and Phip at the mercy of the owlbear if he failed.
Example: Daal, a barbarian who worships the
Nessek (the goddess of fire), and his friends are fighting an ice witch
and her minions. She casts cone of cold on him, and he misses his
saving throw by 1 point. Daal's player knows his character will die if
he takes full damage, so he reminds the DM that while Daal's friends were
recuperating from their last adventure he guarded Nessek's temple for a
week while its champion was away on a quest (the DM had earlier agreed
that this counted as a donation, and priced it based on the daily wage
of a "mercenary leader," so 6 silver pieces per day times 7 days is 42
sp). Daal feel's that the fire goddess might intervene to save him from
a death by cold, and the DM agrees that it might work. The DM rolls percentile,
gets a 25 (sucess!) and retroactively applies a +1 resistance bonus to
Daal's save from a resistance orison granted by the intervention.
Daal makes his saving throw, takes half damage, and is able to go on fighting.