Here are my considerations about religion spreading in the game, and how to actually avoid troubles:
Scenarios: civ A as computer AI civ, civ B as human
civ A and civ B start a game in the same world region
-civ A has no religion of any kind
-civ A converts to the first religion that civ B adopts and spreads around the region
-civ A usually becomes your best friend
-civ A either gets a new religion from someone else, or eventually founds a new one by itself (usually Islam, the latest religion of the game)
-civ A converts and starts spreading its new religion around the continent
-civ A declares war to civ B after it refuses to convert
civ A starts a game on an isolated continent, no contacts with other civs
-civ A discovers theology but doesnt found Christianity, as someone else already did it
-civ A becomes a Theocracy without adopting any religion: it's immune to religion conversion and cannot build missionaries or temples of any kind
-civ A formes its alliances and trade networks
-civ A adopts free religion
-civ A leader is going to be confused about which religion to take- he/she will eventually switch to a new religion everytime a foreign leader will bribe/force him to
-civ A has a confused leader and definitely not a good ally in case of danger
So the key point is that every religion will spread itself around a region where it is originally located- so the only way of blocking an unwanted religion is adopting a theocracy. Of course this doesn't work as a solution, as:
-civ A will probably adopt Free Religion, converting to the first religion to spread in 2/3 of the country
-civ A will found a religion by itself- no matter how many cities were previously under a given religion, an home-developed new religion seems to be more "attractive"? Correct me if I'm wrong
If the human player doesn't give a damn about having 7 different religions in his own country, maybe he actually dislikes watching a Christian continent switching to Islam and allying against him. So far the only solution I found about this was declaring war against the civ discovering the new religion you fear will spread around and convert your allies, then raze its city or origin asap.
Founding the new religion by yourself doesn't actually seem to work, as you may have other things to do that waste research on that, besides the new religion will eventually spread around your cities no matter what you do- you're not going to raze your own city, are you?
Scenarios: civ A as computer AI civ, civ B as human
civ A and civ B start a game in the same world region
-civ A has no religion of any kind
-civ A converts to the first religion that civ B adopts and spreads around the region
-civ A usually becomes your best friend
-civ A either gets a new religion from someone else, or eventually founds a new one by itself (usually Islam, the latest religion of the game)
-civ A converts and starts spreading its new religion around the continent
-civ A declares war to civ B after it refuses to convert
civ A starts a game on an isolated continent, no contacts with other civs
-civ A discovers theology but doesnt found Christianity, as someone else already did it
-civ A becomes a Theocracy without adopting any religion: it's immune to religion conversion and cannot build missionaries or temples of any kind
-civ A formes its alliances and trade networks
-civ A adopts free religion
-civ A leader is going to be confused about which religion to take- he/she will eventually switch to a new religion everytime a foreign leader will bribe/force him to
-civ A has a confused leader and definitely not a good ally in case of danger
So the key point is that every religion will spread itself around a region where it is originally located- so the only way of blocking an unwanted religion is adopting a theocracy. Of course this doesn't work as a solution, as:
-civ A will probably adopt Free Religion, converting to the first religion to spread in 2/3 of the country
-civ A will found a religion by itself- no matter how many cities were previously under a given religion, an home-developed new religion seems to be more "attractive"? Correct me if I'm wrong
If the human player doesn't give a damn about having 7 different religions in his own country, maybe he actually dislikes watching a Christian continent switching to Islam and allying against him. So far the only solution I found about this was declaring war against the civ discovering the new religion you fear will spread around and convert your allies, then raze its city or origin asap.
Founding the new religion by yourself doesn't actually seem to work, as you may have other things to do that waste research on that, besides the new religion will eventually spread around your cities no matter what you do- you're not going to raze your own city, are you?