Christians believed Jesus Christ was the Messiah. They also believed that if they didn't follow the 10 commandments that they would go to "Hell".
The Churches controlled everything! They were corrupt. They made Christians pay for their Indulgences. If you didn't have money you would be Excommunicated.
Martin Luther was not pleased by how they were running the church. Martin Luther wrote the 95 theses about how the church was not good, and put it on the door of the church. He also printed the Bible for everyone so they could read it, the bible was written in Latin.
The Crusades was a major conflict between the Christians and Muslims. The conflict went on and on for about 200 years. Nothing came out of the many years of conflict. Nobody came out on top. They were fighting for their homeland, Jerusalem.