On Towards the Stars: Religious Conversion
[This post is part of a larger strategy guide for Civilization 6]
In a strategy game, a good game system is predictable. A player should be able to understand how their actions will play out if they invest a certain amount of time and resources.
For the longest time, I had no fucking idea how religion worked. I mean, I knew that you had to spam apostles and missionaries, but beyond that all I knew was that sometimes I'd need a single missionary to convert a city, sometimes I need like twenty, and this was only loosely correlated to population. What gives?
Like every other part of civ, religious conversion is based on pooling resources over time. The game calls this resource 'religious pressure', but I like to call them 'conversion points'1. In order to convert a city, you need to have >50% of the total conversion points from all religions accumulated in the city throughout the entire game.
That's it.
There are four ways to add conversion points to a city.
Passive point accumulation. Every turn, cities with a majority religion output a small number of conversion points to all other cities within 10 tiles. 4 conversion points for a holy city, 2 for a city with a holy site, 1 otherwise.
Active accumulation with missionaries and apostles. Should be obvious.
Adding new population. Every new pop adds 50 conversion points for the majority religion (which can include atheism if no majority religion is present).
Trade routes. These give ~1-2 conversion points for the majority religion of the source city to the target city.
Of these, the only one that really matters is missionaries and apostles.
Don't get me wrong, in a game where no one else is really playing a religion, passive growth can be decent. But assuming the city you're trying to convert isn't fully surrounded, you're going to get like max 20 conversion points per turn. By comparison, missionaries and apostles do a burst of conversion points equal to roughly double their health (200 and 220 at full health), and both remove some percentage of opponent conversion points that have built up (10% and 25% respectively). In other words, a single missionary charge is equivalent to >10 turns of passive accommodation. That’s not that great!
You can make passive conversion more viable with Scripture (+25% religious pressure, increases to +50% at Printing) or Itinerant Preachers (30% farther religious spread); of these, I'd take the latter. In the best cases, this increases your conversion points per turn by ten points for a total of 30 conversion points per turn — not necessarily game winning, but definitely useful if you're spreading religion around normally too. Of course, taking either Scripture or Preachers means you don't get the valuable Crusade (+5 combat in foreign cities) or Defenders of the Faith (+3 combat strength in friendly cities) beliefs, which is why it's a mixed bag — faith doesn’t stop tanks.
Anyway, passive behavior aside, you can hopefully already see some important tactics.
First, it's much much easier to convert cities in the early game, before they've had a lot of time to build up conversion points.
Second, you want to spread your early missionaries as far as possible, to as many civs as possible, so you can get passive generation pointing inwards from all sides. Let your opponents help you spread your faith.
Third, again, passive spread just isn't that important because it does not remove conversion points, which missionaries and apostles both do.
Fourth, it's really hard to convert tall cities near the center of big empires, don't bother with missionaries to do that.
But wait, so how do you convert cities that are really heavily entrenched if you can't really use missionaries? Apostles.
Each apostle starts with a free promotion that dramatically changes how you should think about using it. The promotions are listed on the ever resourceful civ wiki.
The most important apostle promotion is Proselytizer — "Religious spread eliminates 75% of existing pressure from other Religions in the target city." This promotion lets you flip even the most stubborn cities, because it effectively brings the total conversion points for the city back down to 0. Pairing a Proselytizer with nearby missionaries (or Translator apostles if you have them) can essentially sweep any civ. Also, Proselytizers work great with Crusade, because you can often flip an edge city with a single charge.
The next most important is Debater — "+20 Religious Strength in Theological Combat". Remember, killing an enemy religious unit results in every nearby city losing religious points for the dead unit and gaining points for your religion. (The tricky part of using debaters is that your opponents can of course just murder your debaters with military units; this promotion works best against the AI)
Finally, Martyr — "Relic is created if this Apostle dies in Theological Combat". This is worth calling out as basically the only way to consistently get relics in the game (and even then it's not easy to have your apostle's die in combat!)
Generally speaking, apostle's get one promotion of three that are randomly selected from a pool of 9 total. That means you have to build a ton of apostles to get strategically useful ones, and you have to really preserve the ones you need. You can make this better with:
Becoming Suzerain of Yerevan, which lets you just choose which promotion you want. This is an incredibly useful religious ability, and it's very worth fighting for suzerainty if you have a religion.
Moksha, who's level 3 promotion gives all Apostle's a second free promotion. The second promotion is sampled from the same pool, meaning you get to "see" up to 6 promotions before selecting 2 (much more likely that you'll get something useful).
Building Mont St. Michel, which gives you the martyr promotion for free. Pretty much only useful if you want a lot of relics, but it's a great way to get there. (Sorta — you still have to actually have your apostles die from theological combat, which can be tricky when most people are just incentivized to kill your units with military)
So if you want to really convert a civ, especially one with a different religion, you'll want to:
build up proselytizers;
send them over as a first wave to lower the total conversion points in each city;
send over other apostles and missionaries as a second wave to flip each city that didn't already flip
And repeat.
IMO the name 'religious pressure' implies that the important part of conversion is the amount of pressure you are exerting in a single snapshot of time, which is not true. 'Conversion points' correctly indicates that the total over time is what's important