Sects or different interpretations of the Bible always existed. Paul's letters show that there were different ideas floating around and Paul tried to unify them.

By the time of the council of Nicaea, there was a big debate among Christian scholars about the relationship of God and Jesus as well as Jesus' nature. Arianism proposed that Jesus was conceived by God and didn't exist since the beginning of time. This was deemed a heresy in the council which tried to unify the different viewpoints.

But arianism didn't die in the council. After some politics, Constantine was baptized by an Arian priest and Arius (the founder of arianism) was exonerated in the council of Tyre and his main opponent was exiled. Arianism was prominently practiced in different parts of the world until the 7th century.

Over time and after the split of the Eastern and Western Roman empire, the doctrine of the church was harder to enforce. Eastern scholars used Greek and Aramaic for their studies while Western ones used Latin. This slowed down idea transfers. This caused, over time, the great schism which split the Western Catholic Church from the Eastern Orthodox Church. The break was over multiple theological differences but also highly political. 250 years earlier, the pope crowned the first Roman emperor in the West, Charlemagne. This practice was discontinued for about 150 until Otto I was crowned emperor of the Roman empire. This was done to increase the papal influence on politics in the West because if the declining influence of Byzantine emperors in the area.

The great schism wasn't the end of heretical groups in Western Europe. Around the time of the schism or shortly thereafter new heresies formed. Two prominent ones are the cathars and the waldensians. This continued until the Reformation started. The waldensians later on joined the calvinists, a newly reformed Church.

The Reformation began after Luther was highly dissatisfied with the church. He saw that the life of the clergy was full of corruption and not at all what Jesus wanted. So he first criticized the church publicly and was punished for it. He was declared vogelfrei (you can do anything to him without being punished yourself) and he was excommunicated. One of his supporters, the prince elector of Saxony, Frederick, "abducted" him and hid him in the castle of Eisenach.

Luther translated the Bible from Hebrew and Greek, instead of from Latin. He also translated the Bible into a form of modern German that could be understood by all German speaking peoples. His efforts were also amplified by the newly invented printing press that allowed for cheap and fast printing of his translation.

This spawned new supporters for his cause and also new interpretations that disagreed with Luther and the Pope. The holy Roman empire experienced a civil was in 1524/25 caused by the anti authoritarian teachings of Luther. England founded its Anglican Church in that time as well.

After another uprising in Germany a more permanent solution was sought. This one was the Peace of Augsburg. In it, rulers were allowed to choose their religion (Lutheranism out Catholic, nothing else) and their subjects had to follow.

This worked until religious and political tensions got too great to endure and in 1618, Protestant Prague citizens threw Representatives of the Catholic Habsburgian emperor out of the window in the second defenestration of Prague. The first one caused a local religious uprising but this one was bigger. This is the start of the 30 years war. A lot of stuff happened, the German countryside was ravaged and death was everywhere. The outcome is there peace of Westphalia in 1648. In it, the religion of each piece of land was defined by the rulers religion but now including calvinism. It also allowed the practice of other religions privately.

After the 30 years war, emigration to America started to take off and many religiously persecuted groups went to seek refugee there. This allowed many denominations to survive and spread. Also new denominations could form.