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religions are split into groups, and any country with a religion that is different from your own, but still in your group, is considered a heretic. You can't declare war on them unless you have a specific national idea. Countries with a different religious group are considered heathens and you can declare "holy war" on them if they share a border with you. Missionaries can convert provinces you own to your state religion, and you can force other countries in your religious group to convert to your religion in peace deals if you have a high enough warscore. You will also occasionally get missions to conquer a Provence that borders you that is of your religion that isn't its owner's religion.
 
religions are split into groups, and any country with a religion that is different from your own, but still in your group, is considered a heretic. You can't declare war on them unless you have a specific national idea. Countries with a different religious group are considered heathens and you can declare "holy war" on them if they share a border with you. Missionaries can convert provinces you own to your state religion, and you can force other countries in your religious group to convert to your religion in peace deals if you have a high enough warscore. You will also occasionally get missions to conquer a Provence that borders you that is of your religion that isn't its owner's religion.

I see, interesting.. Thanks!

http://www.paradoxian.org/eu3wiki/Reference_Guides

Maybe some outdated info but you will get an idea of how it works...

Cheers!

Thank you!
 
The short answer is not as much as it should, yes, yes.

I would recommend that for your first game you play as a western European, in which case you will start out Catholic. This enables you to have cardinals in the Curia (influenced by your number of Catholic provinces and your relations with the Pope). If you have the most cardinals then you become Papal Controller and can influence the Papacy in declaring excommunications and crusades. In about 1500 the reformation will start to spread across Europe as well. Provinces will change religion individually by event and countries may decide to change as well. This gives rise to the possibility of religious wars within Europe. If you are the Holy Roman Emperor for example you can demand religious uniformity and if the country refuses this grants you a Causus Belli to declare war. Up until 1650 Holy Wars are a valid CB against heathen nations. It is granted against any nation that you border and allows for cheap conquest and demanding forced conversion. You can also get this CB through certain government types (Imperial), by National Idea (Unum Sanctum) or by declaring yourself Defender of the Faith which costs about 500 ducats in exchange for some temporary bonuses; all three of these options allow you to declare war regardless of if you border the target country.

The other type of religious war is religious liberation. This applies if you border a country which is 'persecuting' some members of your religion. E.g. An orthodox Balkan state, bordering a Catholic Hungary which has some Orthodox provinces.

Finally missionaries can be used to convert provinces that you own only. They have an initial cost, a monthly cost and a certain chance of success. They cannot fail, the success chance just effects how long it will take them to convert the province. Whilst a missionary is active the revolt risk in the province is significantly raised leading to a chance of having a religious fanatic revolt.

Edited for typos
 
The short answer is not as much as it should, yes, yes.

I would recommend that for your first game you play as a western European, in which case you will start out Catholic. This enables you to have cardinals in the Curia (influenced by your number of Catholic provinces and your relations with the Pope). If you have the most cardinals then you become Papal Controller and can influence the Papacy in declaring excommunications and crusades. In about 1500 the reformation will start to spread across Europe as well. Provinces will change religion individually by event and countries may decide to change as well. This gives rise to the possibility of religious wars within Europe. If you are the Holy Roman Emperor for example you can demand religious uniformity and if the country refuses this grants you a Causus Belli to declare war. Up until 1650 Holy Wars are a valid CB against heathen nations. It is granted against any nation that you border and allows for cheap conquest and demanding forced conversion. You can also get this CB through certain government types (Imperial), by National Idea (Unum Sanctum) or by declaring yourself Defender of the Faith which costs about 500 ducats in exchange for some temporary bonuses; all three of these options allow you to declare war regardless of if you border the target country.

The other type of religious war is religious liberation. This applies if you border a country which is 'persecuting' some members of your religion. E.g. An orthodox Balkan state, bordering a Catholic Hungary which has some Orthodox provinces.

Finally missionaries can be used to convert provinces that you own only. They have an initial cost, a monthly cost and a certain chance of success. They cannot fail, the success chance just effects how long it will take them to convert the province. Whilst a missionary is active the revolt risk in the province is significantly raised leading to a chance of having a religious fanatic revolt.

Edited for typos

Thanks for a great answer.
 
Also christians have a passive yearly colonist while all other religions don't, so expect christians to expand and colonize all over the world after the 1500s.
 
Also christians have a passive yearly colonist while all other religions don't, so expect christians to expand and colonize all over the world after the 1500s.

Heavily affected by your slider settings for province info spreading. If you set the map spread numbers quite high (I generally go with 100 water / 200 land) you will usually see a more historic assortment of nations colonising. With the default settings things get really ridiculous with colonies popping up from most every nation with a port.