Religious Conversion
From Crusaderkings
A religious conversion is the act of a province or vassal changing their religion to yours. Conversions are based on the religion of the ruler at the top of the chain- a Christian Portuguese beneath a Muslim Duke beneath an Orthodox King will eventually convert to Orthodoxy as will his Muslim Duke he serves under.
This can be problematic as the game does not allow the PC to play as non-Christian rulers w/out the use of mods, thus meaning that religious conversion can end one's game, so if you're playing as a Count or Duke it's really not advised that you pledge fealty to a ruler of another religion unless you're both Christian or unless you're playing a mod.
In any case, as you expand your kingdom it's likely that you'll encounter provinces of other religions or that other religions may expand into your backyard.
It should be noted though that wrong-religion provinces will often revolt and have negative events- passing them off to Bishopric or loyal vassal can save you the headache of micromanaging them.
(note- for more information on the exact formulas that modify MTTH, check the religion subfolder in the DB directory of an installed Crusader Kings. Someone add this later.)
In general religious conversions are much easier to pull off than cultural conversions. You don't need complicated trade links being linked together in a realm, you just need for the top head ruler to be the desired religion you want a province to convert to. Religious buildings, Religious Stringency technology, a ruler with Clerical education (the higher the education the better) and Bishoprics all contribute to religious conversions.
For a peaceful conversion, having between 33% and 55% Clergy power is best. Keep Noble power below 20%
However, if one does not wish to convert quite as peacefully- or is simply in a hurry to convert a province- one can also obtain conversions through converting by the sword. Place noble power above 20% and somewhere as high as 60%. Be mindful that his can lead to revolts, although revolts in Crusader Kings tend to be much easier to put down than in other games (especially the Europa series) as the numbers of revolters is tied to a province's income level- and usually revolts happen in looted provinces, and the status of a revolt lowers the income of a province.
However, ultimately it's up to the player how they wish to rule their kingdom and expand their religion. Will you convert by the sword and demand they follow you or simply convince them of the superiority of your faith?
