This chapter of Civilization VI game guide is dedicated to religious city-states in Civ 6. Here, you will find the description of profits received from influences and taking control over those places.
Bonuses for Envoys
For each religious city you get the same bonuses for having a given number of envoys. Remember, that if you have few representatives in different religious cities the bonuses that you get combine.
Number of Envoys
+2 to faith in capital
+2 to faith in each holy site district
Additional +2 to faith in each holy site district
These bonuses are especially useful if you plan on ending a scenario by achieving religious victory but don't forget that other civilizations may also choose this strategy. Thanks to your connections to religious city-states you can compete with other nations even if your own nation doesn't have good bonuses from the chosen type of victory. Remember, that you must have your own religious units to block other players' actions if you choose different tactic than spreading your religion. Holy sites are one of the first districts that you can build so you shouldn't have any problems with getting the bonuses but you have to manage well your envoys. Control the flow of other nations' envoys and try to keep your Suzerain position in each of the cities.
Your Apostle units can choose from any possible promotion instead of receiving a random promotion.
As a strategist, you know the best how to play a scenario and what path of development should your units take. Thanks to this bonus you can have more control over your Apostles' development and get the best profits for your chosen strategy. If you decide to achieve religious victory you must achieve Suzerain status and if you are not interested in this victory, then at least try to make it harder for other players and deprive them of bonuses.
Automatically converts to your religion and exerts pressure for that religion as if it was a Holy City.
A very important city-state in your way to achieve religious victory. If send enough missionaries and apostles then a city will converts to your religion and will exert pressure on other cities in 10 tiles radius which, in greater perspective, allows you to automatically get new believers. If you want to achieve said victory then you should control this city because your enemies won't be able to convert it by their units which will grant you a passive income in the form of new believers while you are busy spreading your faith in other regions of the map.
Receive a relic every time you discover a Natural Wonder and 50% to faith from all relics.
This bonus is mainly strategic and it is hard to determine its usefulness. Your chances for a proper use of bonuses are higher with a bigger map but you have no guarantee that there will be enough Natural Wonders. Of course, you can get bigger faith profits from the relics that you already have but it should firmly addressed that Kandy should be acquired by those players that want to achieve religious victory. In special situations you can reduce your enemies' influence and deprive them of bonuses for possessing relics.
Your builders can now make Colossal Head improvement.
A full description of this improvement can be found in the chapter dedicated to all improvements. If you decide on basing your empire's functionality on this improvement you must, with great caution, control enemies' influence in the city and constantly make your advantage bigger so no one can be a threat to your position as Suzerain.