Civilization IV has seven religions: Buddhism, Christianity, Confucianism, Hinduism, Islam, Judaism and Taoism. They all have the same effects, differing only in what technologies they require and the material needed to speed production of their cathedral (see the table). Also, the later religions (Confucianism, Christianity, Taoism, and Islam) each get a free missionary in their holy city.
| Religion | Required tech | Cathedral Name | 2×speed | |
|---|---|---|---|---|
| File:Buddhism.png | Buddhism | Meditation | Stupa | 
|
| File:Christianity.png | Christianity | Theology | Cathedral | 
|
| File:Confucianism.png | Confucianism | Code of Laws | Academy |
|
| File:Hinduism.png | Hinduism | Polytheism | Mandir | 
|
| File:Islam.png | Islam | Divine Right | Mosque |
|
| File:Judaism.png | Judaism | Monotheism | Synagogue |
|
| File:Taoism.png | Taoism | Philosophy | Pagoda |
|
Founding of Religions and Holy Cities
The first civ to discover the tech listed in the table above will found the given religion in one of its cities. This city will then be the holy city of that religion, represented by a starred religious icon on the world view. The selection of the holy city from among all cities is partly random, but the algorithm will favor cites:
- with few religions
- not the capital
- with high population
A holy city receives +5 
(instead of the standard +1 ). A Great Prophet may construct a Religious Shrine in a religion's holy city. If a holy city is razed, then the religion will continue to exist (provided that it has spread to other cities), but the holy city will be gone for the remainder of the game.
Spread of Religion
Religion can spread in two ways: via missionaries, and automatically. Religions will only spread automatically to cities which do not have any religion present. Furthermore, automatic spread will happen only if there is trade access (not necessarily a trade route, but the potential for one) between the city and the holy city of some religion. Factors that increase the likelihood of automatic spread are low distance to the holy city, and the presence in the holy city of that religion's shrine.
Otherwise, spread of religion is done by way of missionaries. Missionaries are units that civs may build in a city of a particular religion. The missionary may then move to another city, and if its religion is not present there, attempt to spread its religion. The attempt to spread religion will automatically succeed if there is no religion present. For cities with one or more religions already present, there is a random chance that the missionary will fail. This chance is about 9% per religion present for your own cities, or 12% for foreign cities:
| #Religions already present | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Chance of failure (own city) | - | 9% | 18% | 26% | 35% | 43% | 52% |
| Chance of failure (other city) | - | 12% | 24% | 36% | 48% | 60% | 72% |
Effects of Religion in a City
When a city has a religion present, the following effects apply:
- +1
(if state religion, or if no state religion is adopted) - Ability to build a Temple (with Priesthood)
- Ability to build a Monastery (with Meditation)
- Ability to build a Cathedral (with Music)
- +1 (if Free Religion civic is adopted)
State Religion
When a civilization owns one or more cities containing any particular religion, the civ can choose to make that religion its state religion, which, like changing civics, will result in temporary anarchy for leaders who do not possess the Spiritual leader trait. Adopting a state religion gives +1 
to all cities containing that religion and allows access to the effects of religious Civics.
Effects of State Religion in a City
The following effects apply in each city with the state religion present in it:
- +1
- +2 if city has a cathedral of the state religion
- +1 (or +5 for the holy city)
Effects of religious civics:
- +25% , if the city is producing a building or wonder, and running the Organized Religion civic
- a city can build missionaries for any religion present, if running the Organized Religion civic
- +2 experience to all military units produced, if running the Theocracy civic
- +100% great person points produced, if running the Pacifism civic
The following wonders concern state religions:
Other Effects of State Religion
A civ's state religion also has these effects that are not related to cities:
- Better relations with AI opponents who have the same state religion
- Worse relations with AI opponents who have differing state religions
- A civ's state religion on the turn when it completes the Apostolic Palace determines the religion the Apostolic Palace is founded in
Other Religions
- In the Rise of Rome campaign, all empires have their respective pagan religions (Rome, Egypt, Greece, etc.).
- The Omens campaign consists of a survival battle between the Protestants and the Catholics
- Other campaigns include ancient Asian, Mesopotamian, and Mesoamerican pagan religions