For centuries, religion was a key underlying concept that had the power to unite many people and give them a sense of membership. For thousands of years, polytheism was a very popular religious affiliation until the transition to monotheism. The belief in one god was a major change in history, especially with regard to Christianity. The unity of faith of Christians has been an ideal from the beginning. On the other hand, Islam and its followers, Muslims, are united in their beliefs and practices because of Allah, or God. Since the earliest days of Christianity, the main destination for pilgrimage has been the Holy Land and one of the most sacred places. this is Jerusalem – it is said that this is where Jesus was crucified and buried. After the Arabs captured Jerusalem from the Byzantines in 638, Christians were still allowed to travel there because the Muslims were quite tolerant of their visitors. However, in the 11th century, things deteriorated and Christians began to reconquer their Holy Land. In turn, the First Crusade set out to take back what belonged to them. In 1095, Pope Urban II received a letter from the Byzantine emperor Alexios I Comnenus, requesting aid against the Seljuk Turks who had seized lands in Asia Minor, Syria, and Palestine. Even before Urban received the call for help, he had learned that the Church of the Holy Sepulcher had been destroyed by a Muslim ruler from Jerusalem. The church was then rebuilt around 1040. False stories of mistreatment of Christians quickly spread throughout the West. However, Muslims actually took advantage of Christians who came to pray in the Holy Land and were pressured to leave them in peace. Nevertheless, the Pope believed that there were... middle of paper ......witnesses and participants. Princeton, 1921. “Crusader Letters Written from the Holy Land.” » In Translations and Reprints of Original Sources of European History. Dana Carlton Munro, tr. and ed., vol. 1, no. 4. Philadelphia: Department of History, University of Pennsylvania, 1900. Marcus, Jacob. The Jew in the Medieval World: A Sourcebook, 315-1791. New York: JPS, 1938. Munro, Dana C. “Urban and the Crusaders.” Translations and reprints of original sources of European history. Vol 1:2. Philadelphia: University of Pennsylvania, 1895. Phillips, Jonathan. Holy Warriors: A Modern History of the Crusades. New York: Random House, 2009. Riley-Smith, Jonathan. “The mindset of the Crusaders in the East, 1095-1300.” In The Oxford Illustrated History of the Crusades, edited by Jonathan Riley-Smith. Oxford: Oxford University Press, 2001.