Here are my first unrefined thoughts, morning shower inspired, on how to use some mechanics to get into the Western medieval mindset.
Sin Points - These work just like Hit Points. You have a bunch of them. When you sin you lose d6 of them, unless it is a minor sin (-1 point) or a giant-sized one (2d6). When your character dies, their soul goes to heaven (no SP damage), hell (0 SP) or purgatory (anything in between).
Only PCs in Purgatory are subject to Raise Dead type effects. PCs in Heaven may occasionally help out their living pals through some miraculous intervention rule and PCs in Hell may mess with them at the DMs discretion. Those in Purgatory might haunt the party from time to time, especially if PCs don't give them a proper burial and insure their estate is disposed of properly. Perhaps a PC can be moved from purgatory to heaven if the surviving party members build churches and establish monasteries in the deceased's name.
How does SP damage heal? Does it come back like HP if one says ones prayers? Maybe communion is worth 1d6, confession and penance 2d6, absolution restores all SP?
Sinning isn't just the DM yelling 'gotcha!' at bad behavior and docking SPs. PCs can opt to sin to score more XP. Maybe 100xp times level once per session for a Deadly Sin. At chargen each PC rolls a d8 to determine their key sin:
8) Roll again twice, rerolling further rolls of 8, but both sins at half XP value.
In order to earn the XP bonus the sin committed must cause actual havoc in play. For example, a Slothful PC could doze off on guard duty, allowing the wandering monster to attack the party's camp by surprise.
No XP may be earned in this way by folks who are already hellbound (0SP).
A Question of Scale
2 hours ago