From Crusader Kings II Wiki
 Gaining Piety
- Small monthly gain based on Learning skill, increased by Majesty technology.
- Large gain by giving money to holy orders, or allowing relatives to join them.
- Catholics: Buy indulgences from the Pope, or go on a pilgrimage.
- Muslims: Sadaqah charity, observe Ramadan, go on Hajj.
- Pagans: Blot/Jarillo/Ukko Jula/Uzagavenes/Nerge celebration.
- Jews: Observe Passover.
- Granting titles to the clergy, scaled by the rank of the title
- Some events will award Piety, usually at the cost of gold or prestige
- Traits award or cost Piety per month, with Zealous or Humble giving 1 Piety/month and many of the Seven Deadly Sins draining Piety monthly.
- Winning battles and occupations in wars with religous enemies grants Piety, as does winning Holy Wars, Crusades, pagan County Conquests and Subjugations
- Muslims gain +0.2 Piety/month for each mosque in their demesne
- Some church buildings give piety to their holder and his liege(s)
- Execution (except of heathens) and Assassination (if you're caught) will reduce your Piety
- Muslims fighting other Muslims of their faith lose 2 Piety/month
 Using Piety
- Having a lot of Piety makes clergy like you more (at a rate of +1 per 25 Piety to a max of +20)
- Gaining the "Paragon of Virtue" ambition requires amassing 500 Piety.
- Many of the Pope's actions cost Piety:
- Lifting Excommunication
- Duchy claims
- Asking for money
- Hiring a Holy Order costs a lump sum of Piety.
- The Muslim-only County Conquest Casus Belli costs Piety.
- Creating a Kingdom costs 200 Piety.
- Creating an Empire costs 400 Piety.