Religion is core to how Crusader Kings II works. As you can only play as christian characters (unless you bypass that limitation, some info on that later), this installment will be from a christian perspective. I'll be going into things like religious spread and heresy.
As I once again went over the character limit, the summary and the sections on converting provinces, popes, antipopes, and tactics are in a comment below.
The Basics
In Crusader Kings II, every province and character follows a sect, and this can have a major effect on their interactions with other characters. If a province is of a different sect than its liege its revolt risk will increase by 8%; far larger than the 2% penalty from being in the wrong culture group.
From a specific character's perspective, anyone of a different religious group is an infidel, while anyone of a different religion within the same religious sub-group (e.g., Catholic and Cathar Christianity) is a heretic. Infidels get a mutual -30 opinion modifier, heretics a -35 opinion modifier, and people of different sub-groups of the same religion (e.g., Catholic and Orthodox) get a -10 opinion modifier.
This penalty is reduced by the religious flexibility technology; 10% per level. As mentioned in the installment on technology, this only has an effect if the tech level has been achieved in the capital county of the character. Do note that their modifier depends on the tech-level in their capital, not yours, so increasing religious flexibility in your capital will not directly make your heretic/infidel subjects like you more, but it will likely make the tech spread faster to them.
Spread of Heresy - County Level
Religions from different religious groups will only spread via inquisitions. Therefore, if you want a province converted to the One True Faith you and/or your vassals will need to use your Court Chaplains to actively convert the province. Heresy on the other hand can spread on its own. The rate of spread is primarily affected by the moral authority (MA) of the church, and I'll describe how that, and any other factors, affects it. MTTH means "mean time to happen" on a per province basis. The lower the number, the more often it will happen.
Heresy appearing isolated:
- MA between 0 and 20% - MTTH = 5400 months (450 years)
- MA between 20 and 40% - MTTH = 7200 months (600 years)
- MA between 40 and 60% - MTTH = 9600 months (800 years)
- MA between 60 and 80% - MTTH = 19200 months (1600 years)
- MA between 80 and 100% - MTTH = 38400 months (3200 years)
As you can see, the moral authority can make a huge difference. Heresy will appear out of nowhere at a rate slighly above 7 times higher at a moral authority of 0-20 than at 80-100. At a moral authority of 0 to 20%, almost half of your provinces will statistically go heretic over the course of a campaign as the length of the campaign is only slightly below the MTTH (387 years as compared to 450).
However, heresy can also spread from province to province, and this happens at a much higher rate:
- MA between 0 and 20% - MTTH = 675 months (56.25 years)
- MA between 20 and 40% - MTTH = 900 months (75 years)
- MA between 40 and 60% - MTTH = 1200 months (100 years)
- MA between 60 and 80% - MTTH = 2400 months (200 years)
- MA between 80 and 100% - MTTH = 4800 months (400 years)
As you can see, even at its slowest this spread is faster than heresy appearing out of nowhere at its fastest. Further, the MTTH is multiplied by 0.75 if the county borders two or more heretic counties, giving these numbers:
- MA between 0 and 20% - MTTH = 506.25 months (42.1875 years)
- MA between 20 and 40% - MTTH = 675 months (56.25 years)
- MA between 40 and 60% - MTTH = 900 months (75 years)
- MA between 60 and 80% - MTTH = 1800 months (150 years)
- MA between 80 and 100% - MTTH = 3600 months (300 years)
Spread of Heresy - Character Level
Heresy can also spread from counties to characters, and from character to character. Only courtiers can pick up heresies from provinces, but they can then spread it onto other characters. The factors affecting province to character spread are these:
- Cynical and MA between 0 and 20% - MTTH = 50.625 months (4.21875 years)
- Cynical and MA between 20 and 40% - MTTH = 67.5 months (5.625 years)
- Cynical and MA between 40 and 60% - MTTH = 90 months (7.5 years)
- Cynical and MA between 60 and 80% - MTTH = 180 months (15 years)
- Cynical and MA between 80 and 100% - MTTH = 360 months (30 years)
- MA between 0 and 20% - MTTH = 202.5 months (16.875 years)
- MA between 20 and 40% - MTTH = 270 months (22.5 years)
- MA between 40 and 60% - MTTH = 360 months (30 years)
- MA between 60 and 80% - MTTH = 720 months (60 years)
- MA between 80 and 100% - MTTH = 1440 months (120 years)
- Zealous and MA between 0 and 20% - MTTH = 1012.5 months (84.375 years)
- Zealous and MA between 20 and 40% - MTTH = 1350 months (112.5 years)
- Zealous and MA between 40 and 60% - MTTH = 1800 months (150 years)
- Zealous and MA between 60 and 80% - MTTH = 3600 months (300 years)
- Zealous and MA between 80 and 100% - MTTH = 7200 months (600 years)
As you can see, both traits and moral authority can make a huge difference here, with a range from 50 months to 7200.Further we've got, as mentioned earlier, character to character spread. Character to character spread can happen as long as the heretic is capable, 16+, and not a prisoner, and is in the same court as at least one other person that is capable, free, 16+, same religious group, has an opinion of 25+ of the heretic, and is not a heretic. Four traits (that the heretic may have) affect the chance of this happening, as well as the moral authority of the church. When this event triggers it will select a random character that fulfills all the earlier criteria, and fire an event for him/her asking him/her to convert to the heresy.
As the total possibilities are so many, I'll list what the MTTH rather than the entire list:
- Base MTTH of 120 months
- Cynical multiplies it by 10
- Zealous divides it by 4
- Diligent divides it by 2
- Slothful multiplies it by 3
- MA between 0 and 20% multiplies it by 0.5625
- MA between 20 and 40% multiplies it by 0.75
- MA between 60 and 80% multiplies it by 2
- MA between 80 and 100% multiplies it by 4
Thus, we get a range from 8.4375 months (zealous, diligent, and MA below 20%) to 14400 months (cynical, slothful, and MA over 80%)
Now, this will not automatically make the target heretic. Instead, the target will have several choices; embracing the heresy, demanding the heretic converts back (can only be done if (s)he is the heretic's liege, and has a Diplomacy stat of 6+), imprisoning the heretic (has to be the heretic's liege, not a close relative, and have the zealous, wroth, or lunatic trait), or simply saying no.
Without any modifiers, all of these choices are equally likely, but two factors modify the chance of embracing the heresy; being independent halves it, and being a king or emperor quarters it.
Thus, the ruler of an independent realm is unlikely to convert to a heresy, but it can happen.
Do note that while infidel religions cannot spread to your courtiers this way, they can be spread by infidel vassals using their court chaplains. Therefore you should avoid having infidel vassals (and with medium crown authority you can revoke their titles for free anyway)
Converting Provinces
Converting provinces is relatively simple, you just put your court chaplain in the province with the inquisition mission. Your vassals might do this as well. The chance of this happening is modified by several factors, both in the county and on the court chaplain's part:
- Base MTTH of 960 months
- Same culture group multiplies it by 0.75
- Same culture multiplies it by 0.75
- Same religious group multiplies it by 0.5
- Heresy multiplies it by 1.5
- Court Chaplain learning skill of 1 multiplies it by 2.44
- Skill of 2 multiplies it by 1.953
- Skill of 3 multiplies it by 1.5625
- Skill of 4 multiplies it by 1.25
- Skill of 10 multiplies it by 0.75
- Skill of 11 multiplies it by 0.5625
- Skill of 12 multiplies it by 0.42
- Skill of 13 multiplies it by 0.32
- MA between 0 and 20 multiplies it by 8
- MA between 20 and 40 multiplies it by 2
- MA between 60 and 80 multiplies it by 0.75
- MA between 80 and 100 multiplies it by 0.5625
Assuming you've got a decent court chaplain (skill 13 or higher) the MTTH ranges from 48 to 2430 months. MA can change the MTTH by a factor slightly above 14.
Tactics
You should use your court chaplain to spread your religion at all times, as then you'll end up with much fewer rebellions once you lose the 8% revolt risk from 'wrong religion'. This will also increase the MA of the church slightly every time a province converts, and the pope's opinion of you.
Second, you should make sure your court chaplain has a skill of at least 13, or conversion will be slow.
Third, concentrate on heretics before infidels, as your vassals will never convert to an infidel religion, but can convert to a heresy. The heresy penalty is also larger than the infidel penalty.
Fourth, when converting provinces you should concentrate on those of your own culture and/or cultural group first, as these will convert faster.
Fifth, completing crusades is a great way to increase moral authority, as well as gain the Crusader trait (makes all your bishops and the pope love you, and is gained the moment you set foot in the crusade target area), plus gain a few provinces. A 20% increase in moral authority can make a large difference when it comes to the spread of heresy and the One True Faith.
Sixth, if you want to go heretic at some point, appoint an antipope as early as possible. The lower the moral authority of the church, the faster heresy will spread, and the easier a time you'll have converting yourself and your vassals to the heresy.
Seventh, beware of having an antipope for any extended period of time, with the exception of the scenario above. While you'll rake in money for quite a while, as the MA of the church goes down, heresy will become rife, and your vassals will start hating you and the church taxes will dry up as less and less bishops follow your religion.
Eight, don't keep counties of another religion for yourself, give them to a vassal instead. They'll have a court chaplain of their own that can convert their counties, thus speeding up the spread of your religion. Creating and giving out duchies also has the benefit of creating yet another court chaplain. However, you should try to keep your vassal holdings as small as possible, to maximize the number of court chaplains in existence.
Differences Between Orthodox and Catholic
Do note that the orthodox church has no antipopes, cannot call crusades, and all church taxes go to the bishop's liege. Free/papal investiture also does not apply to orthodox factions, you cannot control who takes over when a bishop dies.
This means you don't have to keep your bishops happy for their taxes to go to you, and also that antipopes will never hurt the church's moral authority. However, it also means you'll never gain the benefits of crusades.
Summary
Keep the church strong and you'll reap the benefits of rapid spread of your religion, and hampered heresy. Weaken the church and heresy will be rife, and your religion will be hard to spread. My tips are as follows:
- Antipopes will hurt you in the long run
- Go crusading if you can win
- Prioritize fighting heresy over infidels
- Appoint a good court chaplain
- Create and give out duchies in regions of a different religion
- Divide your vassals
- Focus on counties of your own culture first
- If there are any counties in your realm of a different religion, try to convert them
- Avoid having infidel vassals, they might convert your courtiers
Contributed by Meneth