Piety in Crusader Kings 3 is one of the most important elements of the game. It’s one of the most important currencies in the game, as it will be used for pretty much every action. Even building some holdings and upgrades will require a certain level of Piety. As you rise in the ranks of the monarchy, you will also gain Piety. The more pious you are, it has multiple impacts. Diplomatically, people in your Religion group will be more receptive to you. You may also be able to request favors of your holy figures as well. There are also military options, like going on Crusades or hiring Holy Orders.
There are some negatives though. If you belong to a religious minority hostile to Catholics in the game, a majority of the Catholic religion characters will harbor ill will towards you can be more likely to attack or join in wars against you. Players will be able to use this to their advantage though, as long as they are strategic about who they make mad when. You really don’t want to be the target of a holy war against half of Europe.
To start looking through the Tenets and Doctrines of your religion in Crusader Kings 3, visit the faith page on the lower-left portion of your UI. Click on the Culture icon next to your character portrait to open the faith window. That’s the candle icon in case you can’t find it. This new window should show you what Tenets and Doctrines your faith has active. These will differ from each faith. To gain more Piety in Crusader Kings 3 you need to hold yourself to these laws.
You also get some Piety each month, but certain Traits and Perks can affect this. Having traits that are seen as a Sin, like Wroth, will negatively impact your Piety gain.
The best way to gain Piety in the game right now is to follow your Tenets and Doctrines while also doing things that will please your Churches and vassals. Gaining opinion with religious vassals and lieges can help in this regard. Having more friendly characters means that your overtures when trying to convert people to your religion or succeed in some political aim are more likely to bear fruit.
The more liked you are by your religion seems to correlate with what event chains will spawn for you. Certain times you will notice that random events spawn with relation to religious heads or certain tasks. complete these to raise your Piety. If you start getting too pious, the game may start putting negative events into the picture, just to balance things out.
Various random events will be the most common way to gain small bursts of Piety in the game. The newest sequel is now much more deterministic though in how it generates events. Random events are much more dependent on the life your character leads. The more pious or sinful a character and their past choices, the more likely they are to continue down that path. Always read tooltips for events, they will let you know the chances of certain outcomes.
You may also want to look into the holy sites of your religion, which should be listed on the details page for your faith. Visit these sites to gain a random event that should give you a small piety buff. You cannot go on a pilgrimage when in wartime or while leading armies.