Most of info can be checked on the wiki[ck2.paradoxwikis.com], which I strongly advice to copypaste use.


Feel free to comment if something could be improved, added or is wrong by any means; including grammatically(I'm not english native, obviously)


Images from ck2 wiki[ck2.paradoxwikis.com]
All wrongs reserved.

There are 3 great "families" of religions, and makes some sense to mention that some of their properties are common:

Christianism, Islam and Jewish faiths.

-This are the most philosophically tense religions, having multiple branches(heresies) that divided them. Most of religions only have one heresy related to them but the abrahamic group: Even jewish faith has Samaritan and Karaite as heresies.
In-game this means that if the Moral Authority is low, there're high chances of having not only a single heresy but different arising, making slighty more difficult to unify under a single branch the religion; nonetheless christiandom will be separated by the Roman Church(The Papacy, Catholic), the Coptic Church(The Pope from Alexandria, Miaphysite) the Nestorian Church(Patriarch of the East, Nestorian) and the Bizantine Patriarch(Ecumenical holder of Hagia Sofía, Orthodox) and Islam is splitted into Ibadi, Sunni and Shia.

-The followers of these faiths used to polarice the people through religion, what would/will precipice the Holy Orders and Crusades. The main friction between them was the control of Holy Land: Jerusalem is a holy site for all abrahamic faiths. Undirectly they make pagan reformation a need for those that won't kneel under the cross and the moon.

-They are highly organiced religions, already reformed therefore not modificables by any means(but modding, ofc). This makes their natural Moral Authority higher than the other groups.


Germanic, Slavic, Romuva, Suomenusko, African, Hellenic, Aztec, Bön, Zun and Tengri faiths

-Their followers used to be less impositive than their abrahamic counterparts, so they get along between them pretty well and conversion to a powerfull new paganism or to the local religion is easy and correct. Anyway as we've seen with the abrahamic group, the former polarized the situation due proselytice and impose their beliveings.

-They are not organiced, allowing reformation and locating some of the regional or cultural customs into it: African paganism will inherit the islamic Religious Tax and their cultural predisposition to Equality, Zunism will inherit the Divine Marriage from Zoroastrianism, Germanic paganism the nordic ways, Bön is affine to Han's burocracy and so on. Reformation makes the religion highly customizable.

-Their followers are not forbidden to sack and loot independently of their culture, unless reformed into a Pacifist nature. Pillage is an excelent source of income and prestige, weakens the neighbours and has no down effects.

-There are no philosophical discrepancies until reformation, so there are no heresies and after reforming the only heresy poppable is the old form of religion(Called Old _religion_, iconically)

-The "Offensive/Deffensive Pagan" denomination only points the warmongering or unyelding nature of the unreformed religion, but has no effect at all(And it won't be named again here).


Mazdan, Buddhism, Jainism, Hinduism and Taoism

-It's more a phillosophy than a religion what is translated in-game through different mechanics; the indian group(leaving Taoism as chinesse-influenced, Buddhism as common ground to India, China and Tibet, and Mazdan as persian-influenced) allows changing from one to other just having a county of that religion, Taoism permits the player to dispute or aid China's supremacy...

-Zoroastrianism has heresies(the 4 branches of Mazdan can make a good main religion and have their own set of rules, making them unique rather than regular heresies); the others don't.

• Monastic orders can change religion since they always follow the main religion if heresies exist. Therefore if a Fraticelli Pope usurps The Papacy, Benedictines and Dominicans will be Fraticelli and won't accept Catholics, for instance.
  
  
• Joining the Hermetic Society will always give -10 to theocracies, even if you reformed with Astrology. The only way to prevent it is that the theocratic ruler joined himself(Or you forced him by giving the Apprentice spot). Iqta has no theocracies.
  
  
• Catholicism and all Catholic heresies become Orthodox heresies if the Great Schism is mended. This does not affect the Miaphysite or Nestorian denominations.
  
  
• The Caliphates are titular titles but are destroyed if their holder becomes unlanded and cannot be usurped/claimed from other religion characters.
  
  
• Iqta governments can revoke duchies freely and retract vassals without incurring tyranny.
  
  
• All Muslim types (except Yazidi) do not allow marriages between uncle-niece and aunt-nephew.
  
  
• Muslims cannot adopt Carousing focus unless they are Decadent, Drunkard or Hedonist . Also, AI Muslim characters will never agree to join a party unless they have one of the above traits.
  
  
• Tribal/Nomad muslims don't have Polygamy but Concubinage, so can't ask to Iqta rulers for Concubines.
  
  
• Characters with Islamic Merchant Republic goverment have Polygamy but can't build the Wine Cellar on their Mansions and they're out of the Decadence system.

Every ruler can wage war, he's even allowed to use religion as casus belli declaring a Holy war for a county or duchy making neighbours and vassals from both deffender and offender possibly to take arms for their religion and fellows, but only a head of religion can call for the Crusade/Jihad/Great Holy War(GHW), and this is ment to be a world-wide event making every ruler from the disputed religions to attack/deffend their rightful conquests/lands.
If the Moral Authority is lower than 5% it cannot be wagged.
The start of the Crusades/Jihads is scripted based on this criteria:

Catholics:
-Reclaiming the Holy Land:
Year is at least 1090 AND
Jerusalem not held by Christians
-Byzantium Falling:
Year is at least 900 AND
Byzantine Emperor not Christian OR
Thrace or Ancyra not held by Christians OR
Neither Athens nor Corinth held by Christians
-Fall of Rome:
Year is at least 900 AND
Rome held by non-Christian or by heretic
-France Threatened:
Year is at least 900 AND
Provence, Toulouse, or Paris held by non-Christian
-Germany not controlled:
Year is at least 900 AND
Cologne (Köln) or Braunschweig Christian but held by non-Christian


Islam
-Caliphate Endures
Year is at least 1187 AND
Caliph exists
-Fall of Jerusalem
Year is at least 900 AND
Jerusalem held by non-Muslim
-Fall of Mecca
Year is at least 900 AND
Mecca held by non-Muslim
-Center Collapsing
Year is at least 900 AND
Baghdad, Damascus or Cairo held by non-Muslim
-Africa in Danger
Year is at least 1000 AND
Marrakech, Tunis or Al Djazair (Algiers) held by non-Muslim


Judaism and Mazdan
-Restored religion's head.


Pagans
-ANY pagan faith has been reformed AND either Crusades or Jihads unlocked.


Fraticelli Crusades and Ibadi, Yazidi, Hurufi, and Kharijite Jihads are unlocked once their religious heads are formed, and when the Crusades or Jihads are respectively unlocked.
The Eastern religions and Eastern Christians (Orthodox, Miaphysite and Nestorian) are the only religions without a Great Holy War of any kind.


A religious head can call a GHW if:

The religion's GHW has been unlocked
The religion has not had a GHW end in the last 30 years
Hover the religion icon on any character screen to see when their religion can next call a GHW
Moral authority > 5%
Catholic/Fraticelli cannot call Crusades if the Schism has been mended
During a regency, their council must approve (with DLC icon Conclave.pngConclave)

The target for the GHW must meet the following criteria:

Target is infidel, or heretic vs parent religion, or pagan vs any
Target is not a revolt
Target controls at least 6 counties in the selected de jure kingdom

A ruler can request a GHW if:

At peace
Not at war with religious head
Not a vassal of target realm
No truce with target realm
Can spend 500 piety
Only Christians can request crusades (?)
The Pope will not accept your request if you have free investiture (unless he is your vassal).

Players can request crusades by right-clicking on an independent ruler, selecting "Request Crusade", and then selecting one of the kingdoms controlled by the ruler. If the Pope is a vassal, the liege has to allow external Vassal Wars for the option to appear.


If the principal defender dies or is overthrown the war ends inconclusively.

Holy orders are bands of troops that can be hired with piety to fight religious enemies. Holy orders can also operate as independent rulers, constructing castles and waging their own wars; very much like mercenary vassals.
Most religions only have 1 holy order, Catholics have 5.
Christian holy orders only fight heretics of their religion and infidels. For example, a Catholic holy order will not fight against the Orthodox or Iconoclast Byzantine Empire, but will fight against any Cathar or any Muslim ruler. Muslim, Pagan, and Indian holy orders are also hostile against other religions within their group.

Each Holy Order and his triggers will be included in the religion itself.

Fraticellism is a Catholic Heresy; has identical mechanics to Catholicism including Crusades, but the Fraticelli Pope (if created by a Fraticelli ruler who controls the county of Rome) is a duke-rank title. This means he may be vassalized by a mere king instead of only by an emperor as is the case with the Catholic Pope.

No religious head. Female temple holders. Absolute Cognatic succession. No penalty for female rulers/heirs, councillors and commanders.

Like having Full Status of Woman law or Equality on christianity(So no consorts unless tribal, I guess. Someone can confirm, or I will update soon).

Lack a religious head and crusades. Bishoprics are filled by agnatic open elective.
And thats all. They lose everything from Catholicism except the (regular) Holy War Casus Belli.

Like a reformed faith with Autonomous leadership or christian heresies besides Fraticellism, they have no Religious Head nor Autocephaly.

Similarly to Catharism they allow female temple holders, and there's no penalty for female rulers/heirs but it won't force(or directly allow) Absolute Cognatic for their followers outside theocracies. Also it doesn't allow to appoint female commanders/council members.
No excommunication either, of corse.

The three follow the same mechanics as mainstream Orthodoxy, but have their own always-existent patriarchs, excepting the iconoclasts; they only have the patriarchate created on the 769 Charlemagne start.

An Iconoclast Byzantine Emperor cannot request that the commander of the Varangian Guard convert to it.

Exact same mechanics as Miaphysiteism, the religious head is The Monophysite Papacy.

It has no religious head, like their equalitarian/feminist heretic counterparts, so no excommunication, invasion cb, claims...
Absolute Cognatic succession; female temple holders, councillors and commanders, no penalty for female rulers/heirs and Divine blood, permitting and encouraging marriages to one's own siblings, parents, or children.

They lose the Restoration of Israel events, so can't form a religious head. Everything else is like the regular Judaism.

The religious head is called Sheikh instead of Caliph; The Yazidi Sheikh, and can Excommunicate. It has to be created by decision for 1000 Piety, does not need Sayyid/Mirza trait.
Yazidism has its own unique set of holy sites, Nishapur, Dashtestan, Mosul, Mecca, and Jerusalem.

Got to say that in the reality they're endogamous: If a Yazidi marries a non-Yazidi he is not considered Yazidi anymore. Would've been cool to see that scripted...

They can't create a Caliphate so no Jihads, for everything else is like Sunni(without the schools).

The Caliphate is called The Hurufi Sheikh, but everything else is like Shia. The Caliphate needs Sayyid/Mirza trait as the parent religion.

As Shia, but without religious head thus not able to call for Jihads.

"God is God but is not God, we're muslims but we reincarnate so we're all the Caliph..." and so on. Don't try to understand them, Ck2 and human interest for theologic phillosophy has its own limitations xD

A Kharijite Caliphate can be established for 1000 Piety by a ruler who controls either Mecca and Medina, or Jerusalem, Damascus, and Baghdad. The Sayyid and Mirza traits have no effect. Kharijites otherwise play identically to Ibadis.

They lose the decision to borrow from Jews, Divine Blood and Concubinage. They have their own festival(Decision) and always present religious head(The Yamag), but they can't become Saoshyant.

Manicheists have different Holy Sites than the rest of Mazdan: Khozistan, Baghdad, Samarkand, Jerusalem, Udabhanda.

They have their own Holy Order too:

Mazdaki lack concubines and Divine Blood mechanics or become Saoshyant, but have access to Summer Fair decision. They do not suffer penalties from female rulers and women may hold temples. They can also be appointed as commanders or Moabad.

This religion has the Patron Deity mechanic, similar to Hindus. They may not benefit from Divine Blood or restore the Priesthood but can become Saoshyant.

Myrmidons after reformation. No intel gathered yet.

With the Holy Fury expansion, requirements for reformation remain unchanged, but the player may select four "traits" for religion, which affects the set of available mechanics for the religion and provides various bonuses.


Germanic: +50% retinue size bonus is lost, but +30% levy size bonus remains.

Tengri: No female temple holders. +30% levy size. Light cavalry attack +30%. Light cavalry defense +30%

Aztec: +30% levy size

African: Woman can take up to 3 consorts

Romuva, Slavic, Suomenusko: None

Zunist: Access to Hermetic society

Hellenic: Prohibition to take concubines

Bön: Woman can take up to 3 consorts. Access to monastic feudal government with proper culture


Germanic:
Sons of Ragnarok - Daring(Prepared Invasions) + Sea-Bound(Fleet can navigate through major rivers. Ship maintenance reduced to 10%) - Incompatible with Daring and Sea-Bound doctrines.

Suomenusko:
Survivor of Ukko - Religion is resistant to proselytizing + Stability(Short reign penalty is disabled. Ruler can spend piety to increase courtier attribute) - Incompatible with Stability doctrine

Slavic:
Children of Perun - Fleet can navigate through major rivers, but maintenance cost remains full + Stability(Short reign penalty is disabled. Ruler can spend piety to increase courtier attribute) - Incompatible with Stability doctrine

Tengri:
Eternal Riders - Unrelenting(Ignore defensive attrition. +10% Attack for all units) + Polygamy(Polygamy is allowed. Possibility for woman to take consorts is disallowed even if other traits should unlock it) - Incompatible with Unrelenting and Polygamy doctrine
According to the wiki it won't stack the +10% Attack from Unrelenting due inhereting previous ones.

Romuva:
Defenders of Dievas - Religion is resistant to proselytizing. Eldership Succession law is available - No incompatibilities.

African:
Totem-Guardians - Ruler can order to "Take the Vows". Eldership Succession law is available + Religious Tax - Incompatible with Monasticism and Religious Tax doctrines

Zunism
Dawnbreakers - Divine Marriage(Unlocks divine blood mechanic from Zoroastrism) + Polygamy(Polygamy is allowed. Possibility for woman to take consorts is disallowed even if other traits should unlock it) - Incompatible with Polygamy and Divine Marriage doctrine

Bön:
Harmonious - Meritocracy(Heir Designation is Allowed) + Equality(Absolute cognatic succession is enforced. Full status of woman law is enforced. Woman can take up to 3 consorts) - Incompatible with Agnatic Clans, Enatic Clans, Meritocracy and Equality doctrines

Aztec:
Invaders - Bloodthirsty Gods(Prisoners of different religion can be sacrificed for Piety, Sacrificing prisoners unlocks special traits and actions) + Seabound(Fleet can navigate through major rivers. Ship maintenance reduced to 10%) + Unrelenting(Ignore defensive attrition. +10% Attack for all units) - Incompatible with Bloodthirsty Gods, Sea-Bound, Unrelenting doctrines
Just as Tengri, it will lose the 10% Attack from Unrelenting upon reformation according to what the wiki states.

Hellenic:
Civilized - Ruler do not receive opinion penalties for raised vassal levies + Syncretism(Rulers can choose to gain sympathy with another faith by decisions) + Meritocracy(Heir Designation is Allowed) - Incompatible with Syncretism and Meritocracy doctrine


-Peaceful:

• +/ +3 Demesne limit. +15 Vassal opinion.
  
• +/ +1 piety per month while at peace.
  
• +/ AI rulers are less likely to start wars, plots and factions.
  
  
• -/ Raiding unavailable if culture doesn't allow raiding.
  
• -/ Hunting focus is unavailable.
  
• -/ Great holy wars are unavailable, but county conquest CB is available.

Incompatible with Bloodthirsty Gods, Sea-Bound, Unrelenting doctrines.

-Warmongering:

• +/ Ruler do not receive opinion penalties for raised vassal levies.
  
• -/ Rulers Lose Prestige when at peace for too long.
  
  
• +-/ AI rulers are more agressive.

-Unyielding:

• +/ +20% levy size. +30% garrison size. If there is levy size & attack bonus from unreformed religion, it is lost.
  
• +/ +80% Defense and +80% morale for all units if fighting at county of this religion.
  
• +/ +40% garrison size at county of this religion.
  
• +/ Religion is resistant to proselytizing

Incompatible with Unrelenting doctrine.

-Proselytizing:

• +/ Bonus to missionary missions.
  
  
• -/ Lose piety when at war against faithful.

Incompatible with Bloodthirsty Gods doctrine.

-Dogmatic:

• +/ Bonus to moral authority.
  
  
• -/ Interfaith marriage is unavailable.
  
• -/ Lose piety when at war against faithful.

Incompatible with Syncretism doctrine and Autonomous leadership.

-Cosmopolitan:

• +/ Interfaith marriage is available.
  
  
• -/ Penalty to moral authority and to mission.

(ToDo: Upload images of each one)


-Temporal: The religion has a secular leader(The reformer) as religion head.
The titular duchy title is very much the same as a Caliphate, but may be holded by an unlanded character. It comes with an extra item religion-flavored. +1 Piety if your dinasty controlls it.
Can: Call for GHW. Can't: Excommunicate, grant divorce, grant claim.
On 3.0 before hotfix some conditions can make a random dinastic member to inherit the title, trapping you on it instead just losing it; reforming to merchant republic for instance seems to trigger it.

-Hierocratic: Common for all realms duke-tier religious head. Exactly as a Fraticelli Pope without Cardenal Elections: Can Excommunicate, grant divorces, claims, Invasion Casus Belli and call for GHW.

-Autocephalous:
Each realm has own religious head. Like Orthodox(without a king-tier religious head) or Miaphiste, each religious head is the "Pope" of the realm, so works as Hierocratic with the limitation over realms: You can't ask your religious head to excommunicate another independent ruler of the same religion as he has another head of religion, and so on.

-Autonomous : Incompatible with Dogmatic nature and Ancestor Veneration doctrine
No religious head. 4 different religious branches to choose. Attribute bonus:
+2 Diplomacy for Romuva and Slavic
+2 Martial for Germanic and Tengri
+2 Stewardship for African and Zunist
+2 Intrigue for Suomenusko
+2 Learning for Hellenic and Bön
None for Aztec

No GHW, excommunications, divorces, claims, nor Invasions from (the non-existent) head of religion.
There are no piety markers on the religion tab but buildings will give piety to the holder liege regularly.



-Germanic Reformation:
Since Unyelding won't add all the bonuses it's pointless to select it.
I find Cosmopolitan(For a bigger integration marrying other pagans, mostly) + Sons of Ragnarok + Equality a nice combo, allowing every woman to play just as a man and the chance to have 3 consorts on them... So you can have a regular couple with 3 consorts and 3 concubines all together, very useful to correct the AI marrying their daughters not matrilineally and/or having gigantic families. It's messy for the Gavelkind but can be solved later on, or played fully as a Merchant Republic.
Hierocratic gives you the Fylkyr's Crown, but tend to be buggy and let you get stucked as a random fylkyr courtier, specially as Merchant Republic. Seems stucked to Gavelkind as a Feudal Duke-tier titular title. You can have that and give the title to any other, but you can't give the Crown and won't work without the title... However you can keep the title on the family, not bad.
This way you'll have Conquest CB, HWs and GHs.

• Upload the rest of icons. Add the achievements(with their icons). Waiting confirmation from ck2forums user before uploading Doctrines screenshots.
  
  
• Checking culture-based(Like hunting tiggers as indian) and goverment-based(like cathars allowing consorts as tribal) triggers for each religion.
  
  
• Check some Warrior Lodges(Missing their unique advantages)
  
  
• Add Bloodlines that has boosts/triggers over religion of any kind(Like Karl's and Mehrovech attracting warriors or Attila's general opinion for Tengrists).
  
  
• Religion-based/required artifacts.
  
  
• Upload extra images like holy sites, religious head/holy order/societies icons, etc.
  
  
  Next guide will cover Cultures, Retinues, Goverments, Mercenaries and everything involving these.
  
  
  Show your support with a thumb up and commenting anytime. Allfather Aprobes.