Sid Meier's Civilization V: Gods & Kings/Religion

Religion is a new mechanic introduced in the Gods and Kings expansion. It provides some small but useful benefits to your civs, as well as open up many new strategic choices. While having a religion is not required to achieve any victory condition, a well-made religion can certainly help you reach any of them faster. A religion is composed of multiple layers: a pantheon belief, a founder belief, two follower beliefs and one enhancer belief.

There is a limit to how many religions can be founded, which is one more than half of the default number of civilizations on a set map size. For instance, on a Standard sized map you can only have a maximum of 5 religions, even if you increase the number of civilizations in play over the default 8. Each belief can also be chosen only once, so if you want to have the widest range of beliefs you should found and enhance your religion quickly.

Of course, you don't necessary need to found a religion to win. In fact, it might sometimes be more beneficial to simply follow a religion founded by another civilization. Focusing on improving your religion means you must neglect the development of another aspect. For example, the turns spent to build a Shrine for faith-generation could have been used to build culture-generating Monuments. On the other hand, some situations might require you to found your religion and enhance it early, such as when playing the Byzantiums, as their Unique Ability only provide a benefit if they are a founder of a religion. Whether or not you wish to found a religion depends a lot on your current situation.

Faith
Faith is a new unit of currency similar to production, science, culture and gold. Faith is required for many aspects of religion, from founding one to spreading and defending it. Faith cannot normally be yielded from tiles, they can only come from certain buildings, wonders (natural or man-made), ancient ruins or city-states. It is needed to found a pantheon, found a religion, enhance a religion and buy religious units and buildings.

Pressure
Unlike technologies and social policies, your empire doesn't instantly get the benefits of your religion as soon as you found it, only the holy city from which you've founded your religion will obtain said benefits. However, your cities do exert religious pressure on nearby cities, whether your own or your neighbour's. By default, each city exerts 6 pressure in a range of 10 tiles around them. Once the pressure in a nearby city reaches a certain level, you will gain followers of your religion in that city. The Holy City, where you founded your religion, also exerts a certain amount of pressure to its own citizens, to keep your religion as the majority religion of that city.

Pressure is a purely passive way of spreading a religion. For players who wish to spread their religion more actively, they'll need the help of certain religious units, detailed below.

Religious Units
There are three types of religious units, each dedicated to spreading and defending your religion:


 * Great Prophets: Great Prophets are a type of Great Person and is one of the key steps to founding a religion. Before the Industrial Era, Great Prophets randomly span after certain faith thresholds are reached, starting at 200 faith for your first Great Prophet, but you can freely buy Great Prophets post-Industrial Era using accumulated faith. They can create a Holy Site tile improvement which will make that tile yield 6 faith when worked, found a religion if you can still do so and enhance a religion if you've founded your religion beforehand. They can also spread your religion four times to other cities, as well as pass through the borders of other civilizations without permission. However, they have only 2 mp and are affected by terrain movement penalties.


 * Missionaries: Missionaries are units dedicated to spreading your religion. They have only two conversion spreads compared to the Great Prophet, but are much faster, having 4 mp to the GP's 2. Missionaries must have an Open Borders Agreement to safely move across another civ's borders. If they end a turn in foreign territory without the agreement, they will suffer attrition, which will make them lose conversion strength and outright kill them if it drops to 0. Attrition cannot be reverted, so you must take care not to leave a Missionary inside another civilization's border after a turn ends.


 * Inquisitors: Inquisitors are the opposite of Missionaries, in that they reduce the influence of other religions and protect yours. An Inquisitor within your borders will deter other Great Prophets and Missionaries from trying to convert your citizens. Inquisitors can also purge other religions from one of your cities. Finally, they can also extinguish the Holy City status from a captured enemy Holy City, so that even if the city reconverts to its former religion, it won't have the internal pressure of a Holy City.