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People think of the Middle Ages as a very religious period, when the Christian Church was the most important institution and everybody prayed all the time. Certainly this is the time when the great cathedrals of Europe were built, and also when the Church began the great universities at Paris, Tubingen, Cambridge, and Oxford. This is the time when the Pope might excommunicate a king, and when the king might be very upset about it. Some of the most powerful men and women in the Middle Ages were involved with the Catholic Church.
But what is new about all this is really the idea that religion could have an identity separate from the rest of reality, from the rest of the world. In the ancient world, the gods and their sacrifices were so much a part of everyday life that there is no way to separate religious activities from any other kind of activities. All schools taught about the gods, all meals were sacred to the gods, and all meat was sacrificed to the gods, and pretty much all politicians were also priests, while the Roman emperors were gods themselves. In the Middle Ages, this changes only in that it is now possible to separate the religious from the secular world: both remain very powerful.
One result of identifying religion as a separate thing from politics was that it became important to people that everyone believe the same things that they did. Many people believed that they could only go to Heaven if everybody around them was a good Christian too, so if you believed differently from them, you were forcing them to go to Hell. But in fact Catholicism was not the only form of Christianity during the Middle Ages, and Christianity was not the only faith that people followed either. Most people in Europe and in Turkey were Christians (or, in the 400s and 500s AD, Arians). Most of the people in Western Europe were Catholics, while those of Eastern Europe and Turkey were mostly Orthodox.
Altneue Synagogue in Prague - 1270 AD
But there were also many Jews throughout Europe at this time, and a fair number of Muslims as well. And in the early Middle Ages, many people in the countryside were still following the old Greek and Roman gods.
Because Christians wanted everyone else to be Christian as well, they often got into fights with people of other faiths. Sometimes Christians tried to get rid of all the Jews living near them, either by converting them or by killing them or by making the Jews move somewhere else. Sometimes Christians tried to force people to abandon their old Greek and Roman gods. Sometimes Christians led crusades against the Muslims in Spain and in West Asia, to try to push the Muslims out of Seville and Jerusalem and have Christians living there.
Other times the Christians fought with each other, because they disagreed about the exact details of what a Christian should believe. If you thought someone was a bad Christian, you called her a heretic (HERR-eh-tick), and what she believed was a heresy (HERR-ess-ee).