Religious conversion is the act of a province or character switching to another religion.
Character
Note: when a character converts to another religion, their children below 6 will also usually be converted.
Demand conversion
You may ask your courtier or direct vassal to convert to your religion if:
- You must be at peace to demand conversion from landed vassals
- Unreformed pagans and nomads cannot demand conversion
- Cannot demand conversion from:
- Religious heads, holy orders, or mercenaries such as the Varangian Guard
- Holders of temporary titles (revolt leaders)
- Minors
characters
AI characters will accept if:
- Courtiers with the "True believer" flag will never accept conversion demands (characters generated as Jewish councilors or Court Physicians, former cultists who converted as part of "Openly Adopt Faith")
- Your prisoners will accept in return for release
- Landed vassals will accept with opinion at 36+ (Bön landed vassals require higher opinion)
- Courtiers will accept with opinion at 26+ (Bön courtiers require higher opinion)
However, the character may choose to continue to practice their old religion secretly, which can lead to issues if cults are enabled.
When a ruler accepts conversion, all of their courtiers automatically convert, except those who are 
or did not share the ruler's previous religion. This includes children even if they are being educated in another court.
Demanding conversion results in a -5 "demanded conversion" opinion modifier for 5 years. This is almost always compensated by the removal of an opinion malus for religious differences.
Decisions
A character may convert via conversion decisions to the religion of:
- their liege, if in the same religious group
- their Christian, Muslim, Jewish or Zoroastrian spouse or concubine, if Pagan
- the main religion, if heretic and independent
- the reformed faith, if heretic Pagan
- their capital province (Requires
Rajas of India DLC) - the attacker in a religious war, if moral authority is low and attacker's moral authority is high
- another clan leader in the same realm, if nomadic
Most of these decisions are unavailable to characters, or those that hold religious head titles such as Caliph, and some have additional requirements.
Court Chaplain
Embrace heresy
While researching cultural technologies, a Court Chaplain may adopt a heresy. Their liege may imprison them, or convert to the heresy as well.
Proselytize at home
This mission is mostly intended to convert provinces, but it can also convert characters within the province.
在原始宗教首都传教
More likely:
- Personal factors:
- Sympathy for preacher's religion
- 50+ opinion of spouse/concubine/lover/child of the religion
- Sympathy for preacher's religion
- Realm stability factors:
- Any realm province has the missionary's religion
- All realm provinces have the missionary's religion
- My vassals all love me (25+); this is a good time to convert
- Warfare factors:
- Primary defender in any crusade or holy war
- Powerful neighbor of that religion (twice as powerful)
- Pragmatism factors:
- Already adopting civilized ways: government is not nomadic or tribal
- Powerful rulers want a modern religion: king with 12 realm holdings
- Need it to reform government: meets tribal organization and capital holding requirements to adopt feudalism or found a merchant republic
Less likely:
- Primary attacker in holy war or Viking invasion
(?)
- Historically stubborn culture: Saxon, Baltic, or Finno-Ugric
- Likes alcohol: East Slavic or North Germanic, with Muslim missionary
- Religion doing well: 65% moral authority, or 40% MA and our realm contains 3 holy sites
- Homeland attrition is effective: not Tengri or Aztec; no neighboring realm is an organized religion and has a province with military organization 4
From a Physician-Missionary
A tribal or nomadic ruler who does not have a Court Physician may gain one by event RIP.4100. After 30–60 days, the ruler gets a chance to convert.
From province (heresy)
Courtiers and (non-clergy) rulers can pick up heresies by living in a heretic province. The mean time to happen varies between 50 months and 7200 months, depending on the moral authority of the main religion and the character's traits. Players (but not AI) have the option to decline when the event fires.
- 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)
- 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)
From province (SoA)
With the Sons of Abraham DLC, characters can pick up the province religion, if it's in the same religion group but not the top liege's religion. Heretics and pagans are not eligible. The local religion must have at least the same moral authority as the character's old religion. The character's current location is used, so commanders (including rulers) may convert to a foreign religion this way.
From a courtier (heresy)
A heretic courtier may attempt to convert other courtiers, as long as the liege does not share the heresy. The heretic must be 16+, free, and capable. A random target with the parent religion will be chosen among other courtiers who are 16+, free, have the parent religion, and have an opinion of 25+ of the heretic.
Four traits(that the heretic may have) affect the chance of this happening, as well as the moral authority of the main religion. 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.
- Base MTTH of 120 months
- multiplies it by 10
- divides it by 4
divides it by 2
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
As such, 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. As such, 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. As such, you should avoid having infidel vassals (and with medium crown authority you can revoke their titles for free anyway).
From a courtier (SoA)
With the Sons of Abraham DLC, a courtier in the same religious group (but not the same religion) as their liege may attempt to convert someone. A target will be picked at random among courtiers with the same religion as the liege, or the liege.
All targets can simply accept or refuse, with AI behavior influenced by their opinion of the source and whether they are zealous. Lieges can also imprison. Lieges with 7+ learning can debate for reconversion. Finally, courtiers with 7+ intrigue can refuse and blab to the liege. (The liege can then request reconversion, although without 7+ learning this is more likely to anger the courtier than successfully convert them.)
The MTTH modifiers are similar to the heresy event above, except that this event's MTTH depends on the moral authorities of both religions. Non-heretics are 6x less likely to receive this event, to balance out the relative number of non-heretics.
From a friend
A pair of events makes it possible to convert if at least one character is Christian, Muslim, Jewish, or Zoroastrian.
In event 100180, you notice that your friend is an infidel or heretic. You must be Christian, Muslim, Jewish, or Zoroastrian, and not a heretic. For some reason it requires scaled_wealth=0.5 (this might be a bug). If , it gives an option to convert to your friend's religion.
In event 100181, your friend hit the previous event and chose "try to convert my friend". You have the option to convert to your friend's religion.
You may be able to arrange this with strategic use of 
focuses:
- Use the Scholarship focus or Theology focus to become cynical
- Use the Carousing focus to make a friend with an infidel in your realm
- No infidels in your realm? Find someone unmarried in a wrong-religion court, common in contested areas.
- Give them some money if you want them to trigger the first event.
- Use the Business focus and wait for the "Trade Route" event chain. It always picks a ruler with another religion group, unless none are within diplomatic range. If the other ruler's opinion of you is high, you have a decent chance of becoming friends near the end of the event chain.
Children's education
Every character will be born with the religion of the father in a patrilineal marriage, the mother in a matrilineal marriage or bastardy, or the religion of the captor if born in prison. Educators may get a chance to convert the child, with the rules depending on whether the 
Conclave DLC is enabled.
Without Conclave
The guardian and guardian's employer must have the same religion in order for the guardian to convert the child's religion (and same for culture). The mean times to happen are as follows:
| Guardian traits | Not priest | Priest |
|---|---|---|
| and |
30 months | 6 months |
| or |
60 months | 12 months |
| and |
120 months | 24 months |
| None | ||
| and
| ||
| or |
240 months | 48 months |
| and |
480 months | 96 months |
As you can see, a religion change happens about 50% of the time if the guardian has no traits modifying the chance, as a child generally has a guardian for 10 years. If the guardian is , , and/or a priest it becomes far more likely, while on the other hand if the guardian is and/or it becomes quite unlikely.
So if you want to change the religion of a courtier or prisoner, you should assign a guardian who is a priest with the and/or trait.
With Conclave
With the Conclave DLC, the educator's ability to convert the child is controlled only by whether the child has been given the 
or 
.
For details, see Education (Conclave)#Culture and religion.
From secret religion
You may personally make your secret religion public at any time by right-clicking your portrait. If you have achieved the highest rank in the cult corresponding to your secret religion, you can arrange for all members in your top realm to announce their true faith in unison.
Your character may have a secret religion due to:
Secret conversion decisions
You may spend 250 piety to secretly convert to the religion of:
- A friend, spouse, or concubine
- A demesne county's population
- A demesne county's holy site
- Most holy sites are temples, but this decision requires holding the county.
- You cannot secretly convert to a heresy via holy sites.
As a vassal, you can falsely adopt your liege's or top liege's faith by decision. This costs 100 piety and you continue practicing your old religion in secret. If you already have a secret religion, it only costs 50 piety. Each of your courtiers will decide whether to join; those who do will share both your public religion and your secret religion.
Characters with the trait and religious heads cannot secretly convert by decision.
Options in events
You get an option to secretly convert:
- When a cultist tries to secretly induct you into their religion.
- When your lord spiritual converts to a heresy due to the "research cultural tech" job action. The other options are to imprison them or convert publicly. If you choose to convert secretly, your spiritual gets a choice to make his heresy secret as well.
- When your demesne province converts to a heresy. You must have
Sons of Abraham enabled and currently follow the parent religion. If you choose to secretly convert, you lose 250 piety, though you are allowed to go negative since this is an event rather than a decision. This also secretly converts your spouses and children, except for players and characters who already had a secret religion. - In several SoA event chains, if your liege "demands reconversion" or "debates for reconversion", you have the option to only pretend to accept your liege's religion.
省份
宫廷司祭
The Court Chaplain can convert provinces via the Proselytize mission. The base chance of a conversion has a MTTH of 960 months, but is modified by several factors:
| Criteria | Factor |
|---|---|
| Same culture group | x0.75 |
| Same culture | x0.75 |
| Same religious group | x0.5 |
| Heresy | x1.5 |
Court Chaplain learning attribute:
| Learning | Factor |
|---|---|
| 1 | x2.44 |
| 2 | x1.95 |
| 3 | x1.56 |
| 4 | x1.25 |
| 10 | x0.75 |
| 11 | x0.56 |
| 12 | x0.42 |
| 13 | x0.31 |
Current religion moral authority:
| Moral Authority | Factor |
|---|---|
| Between 0 and 20 | x8 |
| Between 20 and 40 | x2 |
| Between 60 and 80 | x0.75 |
| Between 80 and 100 | x0.56 |
Assuming a decent court chaplain (skill 13 or higher) the MTTH ranges from 48 to 2430 months. Moral Authority (MA) can change the MTTH by a factor slightly above 14.
Additionally, every duke and count will have their own court chaplain, so to speed up conversion it's better to have the more vassal levels with different characters.
Prosperity event
If a prospering province has the wrong culture and religion, it can be flipped through a crown focus event. See Culture#Prosperity event.
Spread of heresy
Religions from different religious groups will only spread via inquisitions. As such, 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. "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)
Thus, 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 (from 1066) 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)
Thus, 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)
Secret communities
Members of secret religious cults may use the "Prepare Grounds" ability in their provinces. When the cult performs a mass conversion (Openly Adopt Faith), these provinces will instantly convert along with the cult members. A player may opt instead to convert to their secret religion alone (Personally Adopt Faith), converting their demesne provinces with prepared grounds.
Monastic Order
Characters at the highest rank in their religion's monastic society may, for 1000 devotion, instantly convert any realm province to their religion. Monastic societies are available for all Christian denominations as well as all Indian religions.
Realm
Some ruler decisions/events also affect vassals and realm provinces.
Convert to Attacker Religion
When a ruler takes the convert to attacker religion decision, the rest of the realm is forced to convert as well. Courtiers, vassals, and the capital province all convert (if they had the ruler's old religion).
Mongol conversion
The first two Mongol hordes, the Golden Horde and the Ilkhanate, can convert to the Catholic, Orthodox, Nestorian, Sunni, or Shia religion by a special event.
The Khagan must be unreformed Tengri and not Zealous. The horde's realm must include a province of the new religion. Finally, the time since horde arrival must be between 40 and 80 years.
The MTTH is about 4 years (per target religion), decreased to 3 years if the capital already has the new religion. With a 40-year window, it is therefore likely to happen as long as the Khagan has not reformed the Tengri faith and is not Zealous.
All characters are informed when the event triggers. The Khagan and all "playable" Tengri characters in the realm convert to the new religion. Their capital provinces also convert.
Mass conversion
Any tribal leader, whose religion is pagan unreformed, neighbouring a realm whose leader's religion is organized - e.g. Christianity, Islam, Eastern religions, reformed pagan faith, etc. - can ask the leader of this realm to help convert the tribal leader's realm to the organized religion.
Strategies
传播你的宗教
Most players will want to spread their religion, and in particular, discourage the spread of heresies.
Appoint a good court chaplain: make sure your court chaplain has a skill of at least 13, or conversion will be slow. He should also have the same culture as the province being converted.
Use your court chaplain to spread your religion at all times, as then you'll end up with much fewer rebellions once you lose revolt risk from 'wrong religion'. Converting provinces also increases your religious head's opinion of you.
Avoid having infidel or heretic vassals. They might convert their provinces, their vassals or even your courtiers. If you have them, be on the lookout to convert them ASAP, especially if they are .
Concentrate on heretics before infidels. Heresies frequently spread from province to province. Vassals will never convert to an infidel religion, but can convert to a local heresy. The heresy opinion penalty (-25) is also larger than the infidel penalty (-20). Finally, if you are Catholic, the pope may become mad at you if you hold a heretic province in your demesne for too long.
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.
Keep moral authority high. High moral authority makes court chaplains more effective at proselytizing, and makes heresies pop up and spread less frequently.
- Declare holy wars and win them.
- Participate in Crusades, and win them.
- If Catholic, avoid the temptation of appointing an antipope.
- Ensure that the religion's holy sites are controlled by members of the faith.
- Conquer a holy site and grant the temple to your religious head if he does not control a holy site temple.
If Jewish or Muslim, try to have vassals. Rulers of these religions will only assign their lord spiritual to proselytize if they have high ai_zeal. Muslim realms also benefit from Zealous rulers as they can never become 
, making it easier to lower decadence to the level which grant benefits.
Be careful of powerful vassals who have openly adopted their secret religion. Since you cannot demand their conversion, the best option is to have religious title revocation and revoke all their titles. Be prepared to fight a civil war. As an alternative, consider waiting: the vassals may use religious title revocation to revoke titles within their sub-realms, causing them to become unpopular with their vassals (who are of your religion). You can help along and plot their demise. Their heirs are unlikely to be "true believers" and you can then demand conversion.
Encouraging heresies
You may find it useful to encourage the spread of heresies, either within your own religion or another religion. If you can handle frequent revolts and your enemies can't, you have a huge advantage over your enemies. This will also make it easier to adopt a heresy yourself, if you want to. Encouraging heresies usually involve lowering the moral authority (MA) of the target religion.
If Catholic, appoint an antipope: This lowers the MA of the Catholic church by 30%, making heresies appear and spread more quickly, while making it difficult for rulers to convert their provinces back.
Help heretic rebels win by distracting your enemies. This also has a large effect on moral authority.
Seize the holy sites of a religion and give them to vassals of another religion: As one fully controlled holy site gives +10% to MA, seizing multiple holy sites can greatly damage MA. Note that seizing Catholic (Rome, Christian Cologne) or Islamic holy sites (Mecca, Jerusalem, Baghdad for Sunni, Damascus for Shia) might trigger early Crusades/Jihads.
Becoming a heretic
As a heretic, you will gain Holy War CBs on just about everyone, while maintaining the ability to invite and intermarry within your religious group.
You can get your heir educated by a heretic in a heretic court. (If there isn't a heretic court, invite a heretic and give them land.) But this is a risky strategy because you must deal with unhappy vassals and neighbors at the already-precarious time of succession.
There are several ways to become a heretic as an adult. If you can fill your court with heretics, perhaps through marriage, one of them might attempt to convert you. When your court chaplain has the "research cultural tech" mission, they can convert to a heresy and suggest that you convert as well. With Rajas of India, you can convert to a heresy by moving your capital to a heretic province and spending 500 piety.
Be aware that not all heresies are identical. Among Catholic heresies, for example, only Cathar has access to gender-equal succession and only Fraticelli can have its own Pope. Furthermore, in your game, some heresies will have higher moral authority and more land than others (both visible in the Religion page of the Ledger).