Catholicism is a broad term for describing specific traditions in the Christian churches in theology and doctrine, liturgy, ethics and spiritually. The term usually refers to Christians and churches, western and eastern, in full communium with the Holy See, usually known as the Catholic Church or the Roman Catholic Church.
Catholicism is one of the oldest branches of Christianity and the one that has had the biggest impact on the history of Europe. The formal head of the religion is the Bishop of Rome - the Pope. Defining features in-game:
Head - The Pope
Holy sites - Köln, Rome, Santiago, Jerusalem, Canterbury
- There is always a Pope, even if Rome falls.
- The Pope can grant divorces.
- The Pope can grant the Invasion Casus Belli.
- The Pope can excommunicate Catholic characters.
- The Pope can call great Crusades.
- Catholic rulers can set up Antipopes.
- Catholic Bishops may pay tax to the Pope rather than their secular liege.
Crusades - Crusades are great holy wars called by the Pope against a particular kingdom title held by a non-Christian. Catholic rulers are notified once a crusade is called and can join it an time while it is in progress. Participants in a crusade can gain the Crusader trait, and if the crusade is won, the participant with the greatest contribution will gain the targeted Kingdom title.
Excommunication - Excommunication is the expulsion of an individual from the church. Characters can ask the Pope or relevant Patriarch to excommunicate a particular individual if they are on good enough terms. Excommunicated individuals can pay a lot of money to lift excommunication, or ask the Pope or relevant Patriarch to lift it if they are on good enough terms. Excommunicated rulers suffer relations penalties, and other Christians can call wars against them with the Excommunicated casus belli.
Antipopes - Antipopes are pretenders to the title of the Pope, and vassal religious rulers can be made Antipopes by Catholic rulers. Creating an Antipope prevents the Pope from excommunicating the controller of the Antipope, and allows the ruler to excommunicate whosoever in his realm he wishes. Bishops who like the Antipope better than the Pope will pay taxes to him instead. Creating an Antipope worsens the relations with the Pope, but the Antipope gets a claim on the Papacy and - if pressed - can become the 'true' Pope.
College of Cardinals (requires Sons of Abraham DLC) - There are nine cardinals, and the Pope is always elected from among their number. Cardinals, however, are not elected; they are picked by the Pope from among his courtiers and the bishops of Europe. The selection is based on many factors; age, piety, opinion, campaign fund, culture (the Pope really likes Italians!), etc. When the Pope dies, the cardinals in turn elect his successor, reasoning in much the same way the Pope does in appointing new cardinals.
Catholicism is a quite major religion in Western Europe, and the Crusade is arguably the most powerful tool in the game. However, one cannot play as the Pope for dynastic reasons, so the main way a ruler taps into this power is becoming influential in the Holy See, in all positions.
Having the Pope like you is a big boon for your realm overall. Having high Piety (Mastermind Theologian, Humble, Chaste, etc.), being the same Culture (by controlling the College of Cardinals, having high Stewardship helps with the campaign), and Papal Investiture are all good ways to get the Pope to like you. The Chaplain's Improve Religious Relations job also helps.
Once you have the Pope's favor, you can get him to do favors for you. You can ask him for cash, get him to excommunicate people (softening them up for a war later) or getting him to grant you Invasions or even Crusades, all for your advantage of course.
Alternatively, you could get Free Investiture, elect Bishops who like you, and set up an Antipope to do your whims. This way, you can force him to Excommunicate bad vassals, and any bishop who doesn't pay cash to you pays cash to your Antipope, who in turn pays cash to you. This has disadvantages though, as all Antipopes will reduce MA by a staggering -30, and the Pope and some of your vassals and neighbors will resent your existence.