Dharper's Article, originally from this thread

From time to time, players ask how the Reformation works. I often give simplified answers, because the truth is - it's complicated. My modding philosophy is that I want to take real, historical events and outcomes, then figure out why they happened and generalize them into an event or mechanic that could occur to any country that meets the same conditions. For this reason, the Netherlands aren't guaranteed to become Reformed, nor is England guaranteed to become Protestant or Spain guaranteed to stay Catholic! However, unless they radically diverge from history - say, by England winning the Hundred Years War, or the Netherlands never falling under foreign control - you often (but not always) get historical results.

How does it work? The system is complex, but basically breaks down to a few rules that it tries to follow. Each is based on the experiences of different countries during the Reformation, from Naples to Ireland and from Spain to Hungary. Together, they achieve plausible but dynamic results...and weird games, yes. It's possible to see Catholic Sweden and Protestant Italy show up in your games. I try to avoid that kind of thing, but I dislike removing all agency from the AI, so you're going to get strange result sometimes (and sometimes they even make sense!). Here are the most important rules:

The Spread of Humanism and Church Corruption Edit

The further north a country is, the more likely it is to convert to Protestant. Where Italy and Spain saw only strife if they turned their backs on the Papacy, England and Sweden had long been estranged from it. They saw clearly the corruption of the Church, and the opportunities from converting.
The Renaissance begins in Rome around 1370, then spreads out of Rome and from Catholic provinces adjacent to Muslim ones (e.g. Spain), one province at a time, until it finally reaches northern Europe in the early 16th century. As it spreads, provinces and countries get a choice of whether to side with the Church or support Humanism (provinces invisibly choose based on church corruption, while countries do so visibly in an event). As it mentions in the event, supporting Humanism will encourage Protestantism later on. The AI is coded to only support humanism if they have suffered church corruption, however - something which happens randomly over time. Because of this, countries close to Rome, or those near Muslims (Italy and Castille) tend to stay Catholic, while those in northern Europe (England and Scandinavia) tend to convert to Protestant. This helps to reflect the trend that northern European nations felt cut off from the Papacy and often suffered more from church corruption than places closer to its center.

Bottom line: if you're in northern Europe, your provinces are going to want to convert (and all your neighbours will).

The 95 Theses and the Printing Press Edit

'Provinces and countries that share a common culture or region with the birthplace of the Reformation tend to convert.' Lutheranism spread across Germany rapidly, in large part thanks to the role of the printing press. Luther's criticisms of the Church were written in the common vernacular, often supported by drawings for the illiterate, and were easy to understand compared to the Latin the Church used to oppose him. Although translations did emerge, they were slower to appear and often came from foreign lands, making Lutheranism more suspect.
The 95 Theses event immediately makes many provinces in the same region or sharing the same culture group far more likely to become Protestant, and the same with countries of that culture group. If the Reformer appears in Wittenburg, then Northern Germany will be likely to become Protestant. If he appears instead in Milan, then northern Italy instead will embrace Lutheranism. The location of the 95 Theses is dynamic but random. The Reformer can appear almost anywhere, but within rules: he can only appear in provinces that have embraced humanism (and so have suffered Church abuses) - which means this rule is unlikely to have a huge effect in most games (but see rule #3) unless you happen to get 'lucky' and end up with him appearing in a place which has mostly sided with the Church, like Brittany or Navarra.

So where will he appear? The most likely place is in the Holy Roman Empire, but not simply because it's the HRE. The game tries to find a province that reflects the historical Wittenburg: a place in which the Elector of Saxony would be likely to side with Luther as a thorn in the side of his liege, the Holy Roman Emperor. In game terms, it looks for a medium-sized country that has suffered Church abuses, does not control cardinals, owes fealty in some way to a more powerful country, and is biased towards appearing within a human player's territory (because the game is, after all, about YOU having fun and experiencing events!)

Bottom line: if the Reformer appears next to you, it will be easier to convert and harder to stay Catholic.

The Appeal to the (German) Nobility Edit

'Most countries won't convert to Protestant until the Peasants War ends. If the Reformer is executed before this, countries will reject Protestantism and become more likely to convert to Reformed instead, if they convert at all. 'The Peasants War was seen by many as the obvious outcome of supporting heresy. When Luther emerged in response to declare the German princes divinely appointed, it was a signal to them that the new heresy could be used by the state - not just as a bargaining chip or by fanatics, but as a tool to centralize power.
This rule is the flip side of rule #2. There are several opportunities for the Reformer to die. He could be simply sent to Rome for trial, or could be placed on trial in his home country, found guilty and executed. His protector might back down and hand him over to the authorities. Or he could be caught by soldiers and dragged from his hiding place in the middle of a war. Either way, if he dies before the Peasants War ends, he will never get the chance to appeal to the nobility. Protestantism will be seen as a radical faith both by rulers (who will not want to convert) and by the population at large (who will see radical revolution as their only way to embrace reform). In this scenario, provinces and countries affected by the Reformer directly (see rule 2) become more likely to stay Catholic, or convert to Reformed if they are going to convert. In these scenarios, Protestantism can end up being a small minority, while Reformed becomes a much larger presence in Europe than it did historically.

Bottom line: if the Reformer dies, expect to see a lot of Reformed provinces in his homeland.

Centralization of Power Edit

Provinces belonging to your state culture are likely to follow your ruler's example. The king of Sweden and Elector of Saxony had few problems converting their people to Lutheranism. However, while the Archduke of Austria was able to force Protestants out of Austria, he failed to keep control over them in Bohemia or Hungary
When the Reformation begins, countries get one last chance to signal which way they wish to go in the Reformation: stay Catholic or embrace Protestantism. When you choose your side, you are also having an invisible but powerful effect on your country: provinces of your primary culture from the 'wrong' side become very likely to support your monarch's decision instead. Later events, such as the Jesuits for Catholics and the Magisterial Reformation for Protestants, follow similar rules - they will spread your state religion within your culture group, but not outside it. For the most part, this means that small, unified states can avoid many troubles at all during the Reformation - but large, multiethnic empires? They follow the French model...which isn't pretty.

Bottom line: Small states can convert freely - go ahead and make Milan Protestant! 

The Birth of Nationalism Edit

'Provinces belonging to a different culture group and those cut off from your capital become likely to oppose you, no matter which religion you choose.' When England converted to Anglican, many Irish Catholics became even more fervent in their religion as a way to distinguish themselves from their hated occupiers. Meanwhile, many Dutch saw the Reformed Church as a way of throwing off the Spanish yoke.
This rule is the flip side of rule #4. Provinces belonging to a different culture group are likely to side against you in the Reformation, while provinces cut off from your capital (exclaves) are also likely. Provinces that fit both criteria become almost certain to do so! There is also a related event which makes small countries likely to convert to Reformed if they border larger Catholic countries that own provinces of their culture, spreading it - and unrest - among their brethren at the same time, in hopes of throwing off your yoke.

Bottom line: Multiethnic empires are going to suffer a lot of religious turmoil, no matter what they choose. Hope you can avoid a War of Religion.

God's Judgment on the Church Edit

'Provinces owned by heathens are likely to embrace the Reformed faith.' The fall of Hungary came as a shock to the Christians living there. Many of them saw their occupation by the Turks as Divine Judgment against the Church - and the Turks had no reason to fight heresy among their heathen subjects as long as they paid their taxes. Calvinism spread rapidly through Hungary, especially in the parts controlled by the Ottomans.
Provinces owned by non-Christians become very likely to convert to Reformed, especially if their owner decides to support the Reformation (which they often do if they rival a Catholic country). Unfortunately, this often doesn't happen in the game - the Turks rarely occupy enough of Europe to have many Catholic provinces - but it's still a rule that occasionally comes up, and in theory could apply in other situations - a resurgent pagan Lithuania, for example. It occasionally also shows up when a Crusader state in Greece converts a province to Catholic, then gets conquered - something I should really fix.

Bottom line: If the Turks conquer Hungary, you'll see a lot of Reformed Hungarians. 

The Radical Reformation Edit

'When countries oppose moderate Protestants, radical Reformed Christians appear instead.' When the Scottish rejected Lutheranism, they sent many Reformers into exile on the continent - where they mingled with Zwingli and Calvin and returned in triumph to drive out the monarchy and institute a Reformed Church of Scotland. Meanwhile, in France, the king's refusal to countenance Lutheranism meant that it was driven underground, where the Huguenots found fertile ground for the Reformed faith.
Protestantism only appears in large numbers where it's wanted, or at least tolerated. When Reformers saw no legitimate way to gain a voice, they sometimes turned to more extremist views. In the 16th century, the works of Zwingli or Calvin were considered radical by many Lutherans (and we haven't even gotten into the Anabaptists!). When countries choose to reject the Reformation entirely, provinces that would ordinarily have spawned Lutheran minorities will instead spawn Reformed ones.

Bottom line: Got humanist provinces and want to avoid Protestants? You'll get Reformed Christians instead.

A Leap of Faith, or the Great Experiment Edit

'Smaller countries, especially republics, are far more likely to convert to Reformed than larger states.' When Jean Calvin was chased out of France, it was the city of Geneva that welcomed him and his radical ideas.
The Reformed faith is more democratic than Lutheranism, which emphasizes the ruler's rightful place as head of the Church. In general, most rulers of 16th century Europe saw the Reformed Church as a heresy that ought to be stomped out. Although many would eventually change their minds, in the beginning major states are very unlikely to convert to Reformed. Instead, smaller countries will do - either because their provinces have involuntarily converted, or because they have suffered a revolt. Once this happens, more countries will join the Reformed faith, but mostly smaller ones which have few provinces to convert. Republics, who already embrace radical new ideas such as voting, are much more likely to allow a radical reformer to preach in their lands, and so become Reformed.

Bottom line: Want to become Reformed early? Change to a republic.