I'm trying to understand the scattered rules for saints and relics, which are split between the core rules, RoP:D and TC. I'm not sure if I'm understanding them correctly, since there are some oddities that make me feel like I'm missing something.
As I understand it, any Christian character may have a patron saint (RoP:D 87). Characters who are particularly pious can have a devotion to multiple saints (TC 13); one of them is "technically" their patron saint, but the choice of which one is your patron seems to have no mechanical impact.
Once per day, you can petition a saint for aid. The text seems to be ambiguous on whether this is once per day total or once per day per saint; I assume it's once per day total since otherwise you could just keep trying different saints until you roll well enough, and that seems like something that would annoy God and the saints themselves.
When you petition a saint you're familiar with (such as your patron saint) you specify the aid you're asking for, which should be one of the miracles listed in the saint's statblock. (When you petition a saint you're unfamiliar with, it seems like you don't usually know what miracles they can grant, and just have to hope that they can do something useful? See RoP:D 87, final paragraph of "invoking a saint".) You then roll a simple die + charm + comm + devotion + modifiers vs 15+ simple die. Modifiers include bonuses for things like being at the saint's tomb or having an urgent need, and big penalties for invoking a saint who you don't have a relationship with or if you received a miracle in the last month.
However, there seems to be no modifier for the might cost of the miracle you're asking for - asking St Peter to repair a broken tool and asking him to resurrect the dead are equally likely to succeed. This seems really weird, and I'm wondering if I've missed something. There's also no limitation on how many people can receive a miracle from a given saint in a day - the normal rules for might pool regeneration don't apply.
According to RoP:D, the -15 penalty for not having an established relationship with the saint applies to all saints except your patron saint. However, TC expands this: you don't take the penalty with any saint who's particularly appropriate to the situation you're facing; for example, when invoking the patron saint of a village you're trying to protect. You also don't take the -15 penalty if you have a relic of the saint.
TC also says that as well as your patron saint, you "may invoke" any saint you are devoted to, and also anyone may invoke Mary. This rule is confusingly written and can be read in two ways, and there's been a lot of debate about which reading is correct. The first reading is that "may invoke" means "may invoke without the -15 penalty". The second option is "may invoke" is literally just that, and by implication you can't invoke any saint at all other than your patron saint, those you're devoted to, and Mary. Under this reading, even for Mary and non-patron saints you're devoted to, you take the -15 penalty unless they're appropriate to the situation. (I personally think the first reading is the correct one, because the same sentence says that one of the saints you're devoted to is "technically" your patron saint, and because getting devotion from a relic explicitly eliminates the -15 penalty.)
If you do not receive a miracle, you can then threaten the saint. This requires a roll of Pr+Leadership+modifiers+stress die vs 9+saint's divine might/5. It does not require you to have anything concrete to threaten the saint with (like threatening to expose the saint's bones), but if you can do that you get a small bonus to your roll. You take the -15 penalty for an unfamiliar saint, but you don't get a bonus for devotion. If you succeed on the roll, you get the miracle; if you fail, then various fun negative consequences can happen depending on how badly you rolled. Again, the might score of the miracle doesn't affect how likely you are to succeed, other than indirectly in that if the saint can grant a high-might miracle then their own divine might is likely to be large.
Have I missed anything?