At some point in the later part of the ancient era, the Roman emperors just up and left Rome for a town named Ravenna because the capital had become dilapidated during the waning years of the empire. That didn't mean people weren't living there anymore, so the pope became the de facto leader of the city; the modern word "pontiff" came from the Latin pontifex, which was just a high-ranking Roman politician. Rome and the pope however were in a precarious position, and between 400 and 550 it was sacked thrice. In the early part of the Middle Ages, this Germanic tribe called the Lombards settled in northern Italy (where we get modern-day "Lombardy"), and caused all sorts of problems for the pope and the people of Rome. After a few hundred years of this shit, Charlemagne came down, subdued the Lombards, and gave a significant chunk of Italy to the pope as a buffer zone around Rome and also allowed him to raise a small army if necessary. We call this the Papal States. The pope held onto it for a millennium until it was taken from him in the mid-1800s during the Italian unification, and all that's left is the Vatican. Even his army is supplied by Switzerland, i.e. the famous Swiss Guard.

The Papal States are a symbol of what went wrong during the Middle Ages and Catholicism. Because the Roman Empire crumbled, the Catholic Church became the government: They were the only literate ones who could write decrees, had the records, or the hierarchical infrastructure to deal with governmental business as barbarian hordes started carving out lands to make kingdoms and roamed around to pillage and loot. Although the church owned land from the moment Christianity became legal and would petition to Constantine for favors, it was never on the scale of the Middle Ages. However it was the Middle Ages and everything was poor and terrible, so it didn't matter that much. It was when we hit the Renaissance and Europe started getting money again that it exploded because popes and bishops were taking that money to make palaces and wear silk clothing.

Not only that, but many of the people who entered the church were not really the type of person who should be there. Nowadays people feel a calling and become a priest or monk or whatever. Back then parents would look at each other and say, "Shit. Well, we have three sons. We have to train all of them to become military men in case the eldest dies and the next one has to take over, but what do we do if all three survive childhood? Well, I guess we could find a wealthy heiress for son number two, but that's already hard enough. What about son number three? I guess throw him into a career for the church?" So you have a kid who grew up with swords on the battlefield and suddenly he was told he's going to live the quiet life. Yeah, that's not going to happen.

Originally the Papal States were supposed to serve as a buffer and originally it worked like that. Eventually it became a domain that had to be defended and even worse, expanded. The infamous Borgias did this; Cesare was slotted to become a bishop but instead turned to the army so he could help his father conquer towns and gain land. How the hell is the holy pope supposed to be a neutral, good party when all he's thinking about is acquiring territory and more worldly power?