Religion
Religion plays a huge role in the game and can determine the outcome of many games. To play such a critical role, it's fairly complex. To make it easier to understand, I will have to break things into a few categories. Information on individual Pantheons can be found
here.
The categories are listed below:
Pantheons (Overview)
Celtic Pantheons (Overview)
Religious Units
Founding, Enhancing and Reforming a Religion
Founder Beliefs
Follower Beliefs
Enhancer Beliefs
Reformation Beliefs
Religious Pressure (Basics)
Fealty Policy (Basics)
Pantheons (Overview):
Pantheons are often the difference between finding and missing out on a religion. While Shrines and some Wonders can help, they still often pale in
Faith generation compared to what you can gain from Pantheons. Since Pantheons vary a lot, they all have their own niches except for a select few. If you followed the link at the start of this post, you will see the various situations they are suited for. Some are designed for lots of
Faith generation with small bonuses while others aren't geared towards finding a religion and instead focuses on getting you ahead. Since only one Pantheon can be obtained (with the exception of Byzantium), there might be situations when you want to prioritize a Pantheon to greatly take advantage of your starting position.
The Celts have their own pool of Pantheons (to be covered below). However, they still function similarly to normal Pantheons in many ways. First, all your current and future Cities adopt the Pantheon as soon as you acquire it. You don't need to spread it. Do note that the followers of your Pantheon will always be fewer than your total number of
Citizen in the City. This will matter for the Pantheons that give yields proportional to the number of Pantheon followers. While this is very unlikely to show up in your game, Pantheons cannot be founded once any religion has been enhanced. This was relevant when consecutive Pantheons cost more
Faith but VP has it so the cost of new Pantheons stay the same.
Celtic Pantheons (Overview):
Celtic Pantheons won't be discussed in detail because it's both numerous and they are in the process of being changed. However, these are a couple of things you need to keep in mind when you are playing as or with Celts in your games.
The Celtic Pantheons belong to its own pool so no other civilization, including Byzantium can obtain it. Due to this, these tend to be stronger than your generic Pantheons. In addition, no one else can enjoy their benefits. When a religion is found, the Pantheon used to find that religion is part of its bonuses. This isn't true for Celtic Pantheons. Therefore, you can hurt your neighbors by spreading your Celtic religion or get hurt when you are converted over to a Celtic religion due to having less bonuses. That's something to keep in mind of. Since the Celtic Pantheons belong to its own pool, Celts never have to worry about competing with others to get the most "optimal" choice and that makes the civilization more versatile in how the player can shape the rest of the game. Other than belonging to its own pool and being exclusive to the Celts, the Celtic Pantheons function like normal Pantheons in other aspects I mentioned above like it's spread automatically to current and future Cities.
Religious Units:
There are three religious units: Great Prophet, Missionary and Inquisitor. They all have their own functions and cost
Faith to purchase.
Great Prophet:
The Great Prophet is necessary to unlocking a religion. At 800
Faith on standard speed, a Great Prophet will spawn in the City that provided you with the 800th
Faith. This means that it's possible to spawn it in a City that isn't your capital which, in most cases, isn't optimal. Note that this is only possible after you got your Pantheon so your first 50
Faith is used for that function. Some UA like the Mayan UA doesn't let you spawn a Great Prophet until you already spawned one through sufficient
Faith. Once this unit is spawned, then you can "Found a Religion" and must make some decisions regarding how you want to shape the rest of your game. After using this action, the Great Prophet is gone and you'll need a new one for any other abilities it possesses.
The Great Prophet can also convert Cities to the religion it belongs to. It's possible for you to have multiple Holy Cities and, if you didn't find any of them, you can gain a Great Prophet for your majority religion. A majority religion is the religion that most of your Cities are converted to. If you captured a few Holy Cities, you can control which religion becomes your majority and spawn Great Prophet for that respective religion. If you wish to speed up your spread, then the Great Prophet's spread action can be very useful. This unit has four spreading charges and its conversion strength doesn't decay in foreign territory. Except for Cities that have a lot of resistance to religious pressure, you can usually convert a single City over with a single conversion charge. While it's much tougher to do so with enemy Holy Cities, two charges tend to be enough even for those stubborn Cities.
Once you found a religion or obtained a religion from your neighbor, then you can use your Great Prophet to enhance it. Of course, there are a few conditions that needs to be met here. First, your Great Prophet must be in the Holy City (I'm not sure if you can enhance while in a different City but the unit spawns in your Holy City anyways and it doesn't make sense to move it). Secondly, you mustn't have used any of the Spread Religion charges. Even using one will remove your option to Enhance as it can be abused by human players who will use all but one charge to maximize one's gain. Like Founding a religion, this action will expend the Great Prophet.
The Great Prophet has one last function which is creating a Holy Site. Holy Sites are improvements that can be placed on any land tiles (not sure about Mountains for Inca) and provides primarily
Faith,
Culture and
Tourism. This expends the Great Prophet but the Holy Site is a very nice improvement to have, especially later in the game when you have already found, enhanced and spread as much as you could your Religion. It's definitely something to consider when you pick which Great Person to spawn starting Industrial Era.
Missionary:
The Missionary is your primary tool of spreading your religion. Missionaries are purchased with
Faith with the cost increasing with each era. They have two charges and start with 1000 Conversion Strength. The Conversion Strength will decay 25% of the base Conversion Strength (250) when your Missionary enters foreign (non City State) territory unless you have Open Borders. A Missionary disappears once its Conversion Strength drops to 0. Compared to a Great Prophet, Missionary have a lot less conversion strength as it's very dependent on various factors like if there are other religions available and the pressure of various religions. However, Missionaries are far easier to purchase early on when Great Prophet are more necessary to enhance a religion. Early in the game, Missionary is the main
Faith sink since other things like buildings are lower on the priority. This is of course assuming that you are playing a peaceful game.
To spread religion, a Missionary (and a Great Prophet as well) must be adjacent or on a City. This isn't possible for City States allied to Spain but this might have been changed due to a rework of Spain. The Missionary converts some Citizens of the City over so, for large Cities with established religion, the number of Citizens is fewer and more charges are needed to convert the City over to your religion. This is also true for City States. As a whole, early game is the best time to spread since no other religion is established and you can use that window of opportunity to make your religion the dominant religion on your continent. An Inquisitor will make the life of a Missionary difficult but more on that later.
Inquisitor:
The Inquisitor is, in some ways, the opposite of a Missionary. The unit is there to slow down enemy spread and/or remove foreign religions so you can make your own religion the dominant one. The Inquisitor was once very powerful as it completely prevents a Missionary and even a Great Prophet from spreading their religion. They are now nerfed greatly but they are still useful units. An Inquisitor costs the same amount of
Faith to purchase and that cost also increases with each era. This unit doesn't become available until a Religion is enhanced so, if you can enhance your religion before your opponent, then you are stronger defensively than your neighbor. Since Inquisitors can only be used in friendly territory, they shouldn't be sent abroad at all.
So, what is the nerfed Inquisitor like? First, it doesn't prevent enemy spreading their religion altogether. It does, however, cut down the effectiveness by half when garrisoned in a City. This 50% effectiveness applies both to active spread and pressure spreading. In other words, you are paying for one Inquisitor to counter one Missionary by forcing it to use both charges to do the same amount of damage it would've done without expending it. Overall, it's a rather powerful passive ability that, even when nerfed, is still quite relevant and cost efficient. Then, there's the Inquisitor's second ability that does expend it and it removes foreign religions when you have majority religion in that City. All foreign religions are removed at a cost of the City "in anarchy" for one turn. This is a great way to deal with the unhappiness caused by having too many different religions and not enough belonging to your dominant religion, especially if your yields scale off your followers.
Founding, Enhancing and Reforming a Religion:
As mentioned above, you need a Great Prophet to found a religion. You cannot found a religion until you have a Pantheon but, thankfully, the game doesn't start spawning a Great Prophet until you get a Pantheon anyways. Once you accumulated enough
Faith, you spawn a Great Prophet and will always found a religion. Note that there are limits to how many religions can be found. On a standard sized map, there can only be 5 religions. Larger sizes allow more religions while smaller sizes make it even more competitive. If you miss out and aren't playing as Byzantium, then you are definitely out of luck. Your only chance at a religion is to take one from your neighbor through force or you might be content on just adopting an existing religion and revolve your gameplay on that.
When founding a religion, you have a few steps you must complete after you click "Found Religion" with your Great Prophet. First, you must pick an icon. Secondly, you can use the existing name or type your own (within whatever character limit it has). Thirdly, you pick a Founder belief. Finally, you pick you Follower belief. You'll see on your screen the Pantheon you picked so that's also part of your religion. Note that, if you don't find a religion, your Pantheon will eventually vanish once all your Cities adopt a religion. If you are playing as Byzantium, you get a Bonus belief as well and this lets you pick from the Pantheon, Founder, Follower and Enhancer pool. You don't get access to the Reformation pool but, even then, it's very powerful and no one is complaining about it. More on Founder and Follower beliefs in the next section.
After founding your religion, your future Great Prophet will be belong to the religion you founded and spawns in your Holy City, oftentimes your Capital. The next Great Prophet is used to enhance your religion, though the timing of when you spawn that Great Prophet depends on your game. Some games might see you spreading your religion more while others might see you focused on enhancing earlier. For enhancing, you choose a second follower belief and an enhancer belief. Note that, with the exception of Byzantium, there can only be one follower belief taken. Since enhancing usually happens after all religions are founded, there will be fewer follower beliefs to pick from. Enhancer beliefs are also limited so enhancing first lets you have first pick and there are some really good options to pick from.
Finally, we get to reforming a religion. This action is very dependent on luck and lots of investment. If you are the only civilization with a religion and have neighbors on your continent, it's the easiest requirement. If you are competing with several other religions, this can be an uphill battle. To reform, you need to meet a certain threshold of followers (20% of the world population) under your religion. The earlier you get those followers, the sooner you can build your religious National Wonder to unlock your Reformation belief since, the later the game goes, the more population all the civilizations have and you need to convert more. Late game also sees religions more established so converting Cities become a tougher task. To reform sooner means you are delaying enhancing sooner so there's the weighing of pros and cons between the two. Reformation beliefs can only be taken once so there will be some serious considerations on what you wish to prioritize.
Founder Beliefs:
I won't be going in great detail here or for any of the beliefs. This is more of an overview of what you are looking at for the beliefs. If there's enough interest, I can share my perspective on the various beliefs in a future post.
Founder beliefs are broken into two parts. One is the National Wonder and the other is the effect. All Founder beliefs have their own National Wonder that can be built once you get 20% of the world population. The more observant people will notice that this coincides with Reformation belief. If the National Wonder is built, you can pick your Reformation belief. If, during the construction of this National Wonder, you no longer have the required follower numbers, then you will be unable to build it. Therefore, you need to spread aggressively to have it built. These National Wonders also provide different yields themselves, make Holy Sites better by giving them more yields and also give you delegates in the World Congress for every 10 Cities following your religion. You
want to get this built as it give your civilization a big boost.
Founder beliefs then have their effect which can be short term, mid term or even long term gains. These gains can be tied to spreading your religion, more yields during WLTKD or Golden Ages or gain yields when certain conditions (like conquering a City or unlocking a new Policy) are met. All of these effects are unique and cater to different playstyles. Some also scales with Cities following your religion so they still emphasis spreading your religion more. Overall, the effects are largely what you focus on for which Founder belief you want. While National Wonders are nice, they still pale when compared to what you can obtain from the effects. Some effects like more yields from WLTKD can also synergize very well with certain Follower beliefs.
Follower Beliefs:
You can pick two follower beliefs total unless you're Byzantium. One is picked when you first find a Religion and the other is unlocked as soon as you enhanced your religion. Usually, all religions have a pick at a Follower belief before one religion enhances and gets to pick again. When facing AI, the Follower belief picked can be a little random so you might find the one you want still available even if you aren't the first to found a religion or even enhance a religion. Follower beliefs also fall into two main camps. One camp gives you a religious building that you can purchase with your
Faith. The other camp is getting yields that either have a condition to be met or scales with the number of your followers. Depending on your strategy and what Follower belief is still available, you can see different combination and trying out something new is part of the fun. Civilizations that follow a religion also enjoys the follower beliefs that come with it.
Enhancer Beliefs:
Enhancer beliefs live up to their names because they provide bonuses that can be felt for the rest of the game. This does require a serious investment as a second Great Prophet is needed. However, the cost is well worth it when you consider the bonuses you get. Enhancer beliefs have a greater variety to suit various situations. For instance, Orthodoxy helps spread your religion passively faster and that could see your religion spread to all your neighbors in the late game. There are also beliefs that scale with number of followers in foreign Cities or foreign Cities following your religion. Some are great for warmongers who doesn't want to bother with stamping out foreign religion (like Syncretism) while others benefit tall peaceful play (like Symbolism). Overall, this will require its own posts due to how varied and it makes the decisions more fun. Enhancer Beliefs also enable Inquisitors so this is vital if defending your religion is a priority.
Reformation Beliefs:
Reformation beliefs, like Enhancer beliefs, also have a large variety and are usually better geared towards getting you a certain victory. They focus on a bunch of playstyles with some noticeably stronger than others. Due to their powerful bonuses, you do have to be wary because other civilizations following your religion will also benefit from it. In addition to making you stronger, you must make sure that your neighbors don't use the bonuses against you. What's interesting is that some of the beliefs, like Crusader Spirit and Defender of the Faith, are designed so you benefit against civilizations of other religions. It adds a whole new dimension on how religion work. Therefore, it's certainly a tough decision on whether you reform or enhance first because you won't have the
Faith to do both and still get first pick.
Religious Pressure (Basics):
Followers of a religion exerts pressure of that religion to other Cities within a certain number of tiles. The more followers you have, the larger the pressure. Citizens in a City has to be affected by a certain amount of pressure for a certain number of turns before they are converted passively (as opposed to actively through Missionaries and Great Prophets). In a way, the pressure is adding something to a pool and, once that pool reaches a certain threshold, a citizen is converted. Since a City can have multiple religions, the math for the pressure can get messy. In the end, the higher pressure tend to come out ahead over a longer period of time. In addition to having Cities with your religion, you can also exert more pressure through Trade Routes, religious buildings like a Church and certain beliefs/policies. India emphasizes religious pressure while the Celts don't receive or generate foreign religious pressure. Given that Missionaries become less efficient in the late game and Great Prophets are used for anything but spreading, religious pressure tends to shape how the religious map look at the end of the game.
Fealty Policy (Basics):
Fealty is a policy tree geared for religion. It has three bonuses that are directly related to religion. First, the opener makes
Faith cost of Buildings, Missionaries and Inquisitors cheaper. That can save you a lot of
Faith in the long run while also allowing you to get more ahead by purchasing more stuff and faster. The opener also provides the Monastery which is a great source of
Food and
Science. The next bonus comes from Organized Religion. It gives +1
Faith to your specialist but, more important, it applies more pressure to nearby Cities without your religion. This is very powerful as it can steadily convert your neighbor with the right beliefs and setup. Finally, there's the bonus for finishing Fealty that provides yield to your Cities with your religion and gives
Tourism modifier for Shared Religion (other civilizations that have your religion as their majority religion). These are powerful bonuses that, while not geared towards any specific victory, can certainly be a boost to your mid-game.
Production
Gold (Plantation), +1 
Movement
CS when attacking.
GAP when you expend a 
Great Person, scaling with Era.
1
Trade Routes
Unhappiness from Urbanization.
Happiness to work with when you consider conquering neighboring Cities. This works quite well for a warmonger strategy but it's not the only benefit. The Barracks give your newly produced units one promotion and that can be a big deal early on when any boost can be a difference between crushing the enemy and a stalemate.
, Hellblazer's: 
