They believed that if you follow the Ten Commandments that you will go to heaven. They believe Jesus is the messiah. Their holy book is the Bible. Jesus's homeland is Jerusalem. Christianity is a monotheistic religion which means belief in one god.
The church controlled everything and was corrupt. They would have people pay for their sins with indulgences. If you couldn't pay then you were excommunicated which means you get kicked out of the church.
Martin Luther disagreed with the church and its corruptness. Martin Luther wrote the ninety five thesis and nailed it to the door of the church. He also printed out the bible and distributed it to the community.
The Crusades was a conflict between the Christian and Muslims. The conflict went 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.