Religion is a key concept in Crusader Kings II, and the different religions and their interactions greatly affect how the game is played. Each character in Crusader Kings II has a religion, and the religion of the player's character determines what mechanics are available to the player.
There are six main religious groups in Crusader Kings II. These are Christian, Muslim, Pagan, Zoroastrian, Indian , and Jewish. Each of these (except Zoroastrian and Jewish) is subdivided into multiple branches, which may in turn have their own sub-branches known as heresies. A list of religions is given below:
- Catholic (Heresies: Cathar, Fraticelli, Waldensian, Lollard)
- Orthodox (Heresies: Bogomilist, Monothelite, Iconoclast, Paulician)
- Miaphysite (Heresy: Monophysite)
- Nestorian (Heresy: Messalian)
By default, Christian characters are playable in Crusader Kings II. Muslim characters require The Sword of Islam DLC to play, and Pagan and Zoroastrian characters require The Old Gods DLC to play. Aztec Pagan characters require Sunset Invasion DLC to play. In addition, Hindu, Buddhist and Jain characters require Rajas of India DLC to play.
Religious Heads: Some of the religions in Crusader Kings II have heads. These people are recognised by the adherents of that particular faith to be the leader of the religion. These individuals may or may not be playable depending on the relgion, and often have unique mechanics. Pagan religions start without a religious head, but gain one once the religion has been reformed. The Norse leader is the Fylkir, a title held by a Norse ruler, which functions much like the Caliphate. The other pagan religions get a normal high priest who is a vassal to the ruler who reformed the religion.
Moral Authority: A religion's moral authority represents the power and sway of a particular religion is measured out of 100%. Certain actions such as excommunications, sanctioned invasions and relgious reformation require a minimum moral authority. Religions with higher moral authority more easily convert characters and provinces to their faith, while religions with lower moral authority are less likely to gain converts and are suscepitible to heresy. A religion's moral authority is dependent on the attributes of the religion's head, the number of holy sites held by the relgion, success in religous wars and other considerations.
Holy Sites: Holy sites are religious holdings in the game with particular significance to a given religion. Holding holy sites gives religions more moral authority. 3 holy sites are required to be held for religious reformation to take place. Some holy sites are significant for multiple religions e.g. Jerusalem.
Great Holy Wars: Catholics, Muslims, Reformed Pagans, Zoroastrians, and Jews gain access to a special CB that allows them to fight for all of the territories in an entire kingdom to win them to the faith. The name of this CB changes by religion: Catholics have crusades; Muslims have jihads; the rest have great holy wars. Crusades and jihads are unlocked in the 11th century or in the case of major centers of religion to infidels or heretics, i.e. Rome falling to Muslim invaders. Once both crusades and jihads are unlocked, the reformed pagans will gain access to great holy wars. Zoroastrians and Jews immediately gain access to great holy wars upon restoring their priesthoods. Once a great holy war has been called, all members of that religion may join the war. The one whose war contribution is the greatest is awared control of the target territory if successful.
Heresies: Each religion has one or more heresies, which function as subgroups. Heresies are especially likely to appear and spread rapidly if the moral authority of the parent religion has been eroded considerably. Their spread is problematic because the relations penalty between heretics and members of the parent religion is severe and thus fosters feudal strife. Players can, however, choose to become heretics themselves. Heretics and their parent religions gain holy war CBs on each other. While this eases expansion for heretic lords surrounded by former co-religionists, heretics do not gain access to holy orders. Should the heretic religion become more widespread than the parent religion, the heresy will displace it and become the new orthodoxy. The holy orders will then serve the former heresy.
Pagan Reformation: Pagan religions begin without religious heads or access great holy wars or holy orders. Low-tech non-pagans gain a significant attrition penalty when fighting in pagan territory, but pagan religions have significant penalty to conversion and are more easily displaced by their organized neighbors. Unreformed pagan rulers also cannot force vassals to convert to their religion. A pagan ruler can reform the religion, giving it an organized clergy and holy texts, by controlling holy sites. Once the religion is reformed, it will gain a religious head, a holy order, and access to the Great Holy War CB once the crusades and jihads have begun. In the case of the Norse, the religious head is the reforming ruler and the mechanics function much like those of the Islamic Caliphate; for the rest, the religious heads are simply vassals of the ruler. Reformed pagans lose all the penalties and bonuses of the unreformed religion.
For more information on the different religions and their unique mechanics see their individual pages.
For an indepth religion strategy guide see: Religion & Keeping the Church Strong
To learn about popes and antipopes see: Pope (& Antipope)