Religion

§ Religion has always played a critical part in human history. Through religion, man has sought to make sense of the universe around him and to determine his place in it. Religion has inspired, enlightened and ennobled man; in its name men have erected beautiful buildings, written books of great wisdom, and made music of surpassing beauty. In its name men have also murdered and enslaved their fellows.
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Religion in this Game

§ We know that people have extremely strong opinions about religions - in fact, many a war has arisen when these beliefs collide. We at Firaxis have no desire to offend anyone. However, given the importance that religions have had in human development, we didn't want to simply leave them out of the game altogether; instead we have tried to handle them in as respectful, fair and even-handed manner as possible. In fact, all religions in the game have the same effects, the only difference being their requirements.
§ There are seven religions in Civilization IV (testing having determined that seven was the optimal number for gameplay). When determining which seven to include, we picked those religions that we thought would be most familiar to our audience. We do not mean to imply that these religions are more important, better or worse than any other religions.
§ We offer no value judgments on religion; we mean no disrespect to anyone's beliefs. We're game designers, not theologians.
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Founding a Religion

§ If you are the first civilization to discover the associated technology, the religion is founded in one of your cities. It is extremely useful to have a city where a religion has been founded; it allows a prophet to create a "shrine" in that city (see below).
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State Religions

§ When any cities in your civilization possess one or more religions, you may choose to assign one as the "state" religion. Having a state religion tends to make your cities possessing that religion happier and more productive; it also increases the effects of many religious buildings and religious civics.
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Spreading Religions

§ Religions spread randomly from city to city. Religions tend to spread from a city possessing the religion to nearby cities as well as to cities the possessing city has trade routes with. Some religious buildings (see below) increase the speed at which a religion will spread, and a civilization's religious civic (see below) can have a great effect upon the spread of religions, as well. Missionaries allow you to control the spread of religion; see the following section.
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Missionaries

§ Civilizations can construct special "missionary" units to spread the civilization's state religion. Each civilization may have only a limited number of missionary units in play at any one time, and each costs two gold per turn to maintain.
§ Building Missionaries
§  Civilizations using the Organized Religion civic can construct missionaries in any city with the state religion; if a civilization is not following Organized Religion, it can build missionaries only in cities with Monasteries.
§ Moving Missionaries
§ Once constructed, missionaries can move about the map like any other unit. Your missionaries can freely enter your territory or neutral territory; they cannot enter another civilization's territory until you have an Open Borders agreement with that civ.
§ How Missionaries Spread Religion
§ When the missionary is in the target city, click on the "spread religion" order in the missionary's unit box (see Units); if the attempt is successful the missionary's religion is now in the target city. Religions spread more easily into other cities within the missionary's civilization. Further, it is easier to spread a religion into a city with few or no religions already present; each resident religion decreases the odds of success. There is no limit to how many religions may be present in a single city; all seven religions can coexist in the same city. The missionary is consumed in the conversion attempt, successful or not.
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Benefits of Religion

§ City Benefits
§ All cities that possess your state religion receive one extra happy face and produce one culture per turn. Spreading your state religion is thus a good way to expand cultural borders in the early stages of your civilization. The holy city for your state religion also receives an additional bonus of five culture per turn, allowing it to expand its borders very quickly.
§ Buildings
§ Without a religion in a city, that city cannot construct religious buildings like Temples and Cathedrals (see below). Cities without religions also cannot create Priest Specialists.
§ Espionage
§ If you control the holy city for a religion, you gain line-of-sight benefits in all foreign cities sharing your religion. This allows you to spy on what enemy cities are doing at no cost.
§ Shrine Income
§ If you control the holy city for a religion and generate a Great Prophet, you can build the Shrine for that religion to generate income for every city in the world that possesses your religion (see below).
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Religious Civics

§ The religious Civics describe how religion affects your civilization. There are five religious civics in the game. You begin the game knowing only paganism; the other four become available when you learn their associated technologies.
§ Paganism: No technology requirement. The basic religious civic.
§ Organized Religion: Requires Monotheism. Increases production of buildings in cities with the state religion, and allows you to build missionaries without first constructing monasteries.
§ Theocracy: Requires Theology. Theocracy grants two additional experience points to units trained within cities with the state religion, and it stops the spread of non-state religions within the civilization.
§ Pacifism: Requires Philosophy. Greatly increases the Great Person birth rate in cities with the state religion, and it increases the support cost of military units.
§ Free Religion: Requires Liberalism. A civilization with Free Religion has no state religion. Each city gets happiness for each non-state religion in the city; also, all cities get a 10% boost in scientific Research.
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Religious Buildings

§ There are a number of religious buildings available to be constructed. Religious buildings are dedicated to specific religions and can only be built in cities which have that religion. A city with more than one religion can have religious buildings for each religion: for example, a city with Judaism and Confucianism could construct both a Jewish Temple and a Confucian Temple.
§ Temples
§ The most primitive religious building. Temples boost a city's Culture and Happiness. They allow 1 population point to be turned into a priest Specialist.
§ Monasteries
§ Monasteries increase a city's research by a small amount. More importantly, they allow the city to build missionaries to spread your religion.
§ Cathedrals
§ Cathedrals drastically increase a city's culture. They increase happiness if their religion is the civilization's state religion. They allow two population points to be turned into priests. Note that "Cathedral" is a generic name for a religion's grander houses of worship - Jewish synagogues, Islamic mosques, Christian cathedrals, etc., all have the same effects. Your civilization can only build one cathedral for every three temples that your civilization possesses; for example, a civilization with seven temples could build two cathedrals.
§ Shrines
§ Shrines can be built only in cities in which a religion has been founded. Shrines are constructed by Great Prophets; a great prophet is the only unit that can build a shrine, and the prophet is consumed in the process. Shrines increase the spread of the religion, as well as the home city's culture. They increase the city's chance of generating prophets and allow the city to assign three population as priests. Most importantly, shrines generate one gold each turn for every city on the map that has the shrine's religion (thus explaining the importance of creating missionaries to spread the word). Each religion has a different name for its cathedral and shrine.