I just finished reading the penultimate chapter of the fourth Harry Potter book. In that series, there is a major threat named Voldemort. Most of the characters oppose him, but they vastly disagree on how to deal with him. Some are so stubborn to accept his return that they paint both Harry and Dumbledore to be senile or attention-seeking.
In the 1200s, sects outside of the Church started to form. All true Christians oppose heresy and the devil. However, we do not all agree on how to react to it or which people are the good guys. It makes people in higher powers go to extremes in cutting off a nose to spite a face.
The Cathari or Albigenses were the first major zit to crop up on the face of the HRE. Their one defense: they did not accept that the Pope was the vicar of Christ. He was just a steward. They also wanted to go back to the New Testament Church. Sadly, they were also Gnostics. They believed in the good good and the bad god. The bad god created matter, and all matter is evil. They opposed reproduction, sacraments (especially Mass), hell, and a physical resurrection. Their only salvation was for this world. Their religion was pointless.
Then, there are people I side with: the Waldensians. In 1176, Peter Waldo read the New Testament, was captivated by Christ, and gave up all his property except enough to feed his family. His followers called themselves the "poor men." They would try to preach in the streets in non-Latin and promoted a Bible for all people in their own languages, the same Bible that should be the final authority in faith and life. In 1184, they were excommunicated. They still continued to preach in the vernacular languages and they still exist in Northern Italy. I agree with the theology of these guys. People no longer spoke Latin, and the Church clung to it the same way people will cling to the KJV today. The people were perishing for lack of the life-giving Scriptures that make people alive to the Gospel.
I do not really agree with starting a movement outside the Church, but working within the Church to solve its issues. I don't believe in people being preachers without Church training. I'd agree with these guys on theology and social issues, but I would disagree with them on their methods of communication.
Lastly, some nut named Joachim developed a bizarre eschatology. God the Father was important in Old Testament times. The Son was important from the New Testament until 1260. After 1260, there will be a time of antichrist, and then a new era will arrive with the Holy Spirit and an age of love. I'll sooner go with dispensational theology than divide the Trinity like that and place a date on end-times when Jesus didn't even give one.
The Church reacted in various ways. True believers in the Dominican Friars tried to reach out to the Cathari. Other folks went extreme and began to exterminate the Albigensians. The Inquisition developed, and now one could not remotely disagree with the Pope without fearing for their lives. Worst of all, the Synod of Toulous in 1229 forbid the Bible in the vernacular languages. Now, the people had to endure the Gospel shrouded in Latin, and they had to accept everything the priests said unchecked and fell into error. The good news is that forbidding books causes them to be popular. So this only started a ripple of what would later become the Reformation.