Religion spreads two ways in Old World. World Religions spread passively via a per-turn % chance and actively via Disciple units. Pagan Religions spread only by building Shrines.
Passive Spread — World Religions
Each turn, every World Religion rolls a specific % chance to spread
to one random City or Tribal Site. The base chance comes from
iSpreadPercent in religion.xml:
| Religion | Base %/turn | Cost (base + per change) | Founding prereqs |
|---|---|---|---|
| Zoroastrianism | 5% | 400 + 200 Civics | 2× Acolyte |
| Judaism | 5% | 400 + 200 Civics | 2× Rancher |
| Christianity | 10% | 400 + 200 Civics | METAPHYSICS12 Citizens (city)3 cities w/ Judaism |
| Manichaeism | 10% | 400 + 200 Civics | MONASTICISM2 cities w/ Zoroastrianism2 cities w/ Christianity |
| Hinduism | 0% | 400 + 200 Civics | No prereqs |
| Buddhism | 10% | 400 + 200 Civics | PHILOSOPHY4 Theologies |
A city is more likely to convert if it is close to the religion's
Holy City and less likely if it already has
one or more religions present. Pagan religions don't roll passive spread
(bNoSpread=1); they require a new Shrine on each city.
Active Spread — Disciples & Shrines
Disciple (World Religions)
Disciples can spread their religion to a settlement, build the
religion's Monastery, 
Temple,
Cathedral, and Holy Site,
and (with Orthodoxy) Purge rival religions.
A Disciple can only be trained in a city of its religion if the city is the Holy City, the religion is your State Religion, or the nation has Tolerance. Disciples are also used to establish Theologies.
Shrine (Pagan Religions)
Each nation has 4 unique Pagan Shrines drawn from a pool of 11 types (War, Kingship, Wisdom, …). Building the first shrine founds the nation's pagan religion; each subsequent shrine in another city spreads it. See the full Shrines table.
Polytheism removes the one-shrine-per-city limit. Divine Rule lets a pagan religion serve as your State Religion.
Theology Spread Modifiers
Establishing a Theology can permanently shift a World Religion's per-turn spread chance, globally and for all followers. Effects compound across tiers — pick aggressive theologies if you intend to evangelize.
| Theology | Tier | Spread % |
|---|---|---|
| Revelation | Tier 2 | +20% |
| Veneration | Tier 1 | +10% |
| Legalism | Tier 1 | +5% |
| Mythology | Tier 1 | +5% |
| Dualism | Tier 2 | +5% |
| Gnosticism | Tier 2 | +5% |
| Enlightenment | Tier 3 | +5% |
| Redemption | Tier 3 | +5% |
State Religion
You don't start with a state religion. Adopting one grants its
followers higher opinion, adds +1 
Happiness in
every city of that religion, and (for World Religions) enables
Disciple training there. Switching costs
iCostBase + iCostPerChange per previous
switch, per religion.
Pagan religions can't be a state religion by default — you need the
Divine Rule law to enact a pagan state religion
(see bPaganStateReligion in effectPlayer.xml).
Worked example — Christianity spreads to Jerusalem
-
Christianity is founded by the first nation with 3 Jewish cities + the
Metaphysics tech. The founding city becomes its
Holy City.
- Each turn that nation's Christianity rolls a 10% chance to spread to one random City or Tribal Site (modified by distance to the Holy City and by theologies like Revelation at +20%).
- Once a city has Christianity, it can train Disciples (provided the city qualifies — see above) which can spread Christianity deterministically to new settlements and build religion buildings.
- With the religion as State Religion, the player can establish a Theology (e.g., Revelation) — which then applies globally to every Christian player and locks that theology slot for the rest of the game.
Family Conversion
Source: Player.cs::doFamilyReligion. Starting turn
10, each turn the game evaluates one
family slot — family i is checked when
turn % 4 == i % 4,
so every family is reconsidered once every 4 turns.
Each religion is scored; a religion must score > 100
to be eligible. Highest score wins (deterministic — the +50 current-religion
bonus is the tiebreaker that keeps families from praying to a different god every cycle).
| Points | Source |
|---|---|
| +100 | Each family character following that religion |
| +100 | Family head (additional, on top of the +100 above → +200 total) |
| +20 | Council members (additional — stacks with the job bonus → +40 for a council job) |
| +20 | Has a job — governor / general / agent / council (additional) |
| +50 | Current religion bonus (tiebreaker so families don't churn) |
Character Conversion
Source: Character.cs::doReligion (the trigger gate at line 7950)
and ::chooseReligion (the scoring at line 8509). Global constants
live in globalsInt.xml.
Who rolls
- Leader characters of human players never auto-convert (`isHuman() && isLeader()`).
- Anyone who already has a religion is skipped (`hasReligion()` check).
- Religion heads, characters with NoReligion trait, and anyone under age 10 (TUTORS_AGE) are ineligible.
- Non-leaders with a 'locked religion' trait can only convert to that locked religion.
- No conversion before turn 10 of the
game (CHARACTER_RELIGION_DELAY_TURNS). - Per turn, each eligible character rolls a 10% chance (CHARACTER_RELIGION_PROB).
- Adults always proceed after the 10% gate. Children (age < 18) proceed with probability 1 / (ADULT_AGE − age + 1). At age 10 that's 1/9; at 17, 1/2. Adulthood = 18.
Eligibility threshold
A random integer 1..3 is drawn at the start of evaluation. Each religion's city-based + role-based score must be **strictly greater than** the threshold to be eligible. The highest post-modifier score among eligible religions wins; ties go nowhere.
City points — with family
Family characters: only their family's cities (player-owned and family-aligned) count.
| +4 | Family seat is the holy city of this religion |
| +3 | Any other family city is the holy city |
| +2 | Religion present in family seat (not as holy city) |
| +1 | Religion present in other family cities (not as holy city) |
City points — no family
Characters without a family (initial rulers, foreigners, etc.). All player cities count, weighted less.
| +2 | Player owns the holy city of this religion |
| +1 | Religion present in any other player city |
City points — tribe character
Tribe characters score off tribe settlements (cities + tribe sites).
| +4 | Tribe city is the holy city |
| +3 | Tribe city has the religion (not holy) |
| +2 | Tribe site has the religion |
Role points
Role bonuses stack on top of city/family scoring, regardless of family affiliation.
| +3 | Governor of the holy city |
| +2 | Governor of a city with this religion (not the holy city) |
| +2 | Agent in the holy city |
| +1 | Agent in a city with this religion (not the holy city) |
Post-threshold modifiers
Only religions whose pre-mod score cleared the threshold get these. Then the random multiplier picks the winner.
| +3 | Religion is a Pagan religion (has a `mePaganNation`) |
| +0–4 | max(0, 4 − total_characters_following) — boosts small religions |
Final tiebreaker
Final score = sum × random integer in [5, 14]. Highest score wins.
Globals (globalsInt.xml): roll probability
10%; conversion starts turn
10; minimum age
10; adulthood at 18.