The holy book for Christianity is called the bible. It contains all the basic ideas of Christianity. Christianity is a monotheistic religion. This means that Christians only believe in one god.
During the middle ages the church had all the power. The church made people believe that salvation came through the church. Not following the rules or questioning the church led to excommunication. You would go to hell.
The church controlled who read the bible. Once Martin read it for himself he realized how corrupted the church was. He wrote the 95 thesis. It was a list of all the ways the church was corrupt.
The crusades was a war between the Muslims and the Christian. They didn't believe in each others religions. This caused tension and led to conflict. Nobody won the conflict however it resulted in trade.