As you may recall, the period of the eleventh and twelfth centuries were when the Crusades took place in the Holy Land. The Roman Catholic Church was on the rebound from serious political and moral reform due to the serious amounts of corruption that existed in their papal offices. As such, the Crusades was a time-period in which they attempted to strengthen their reputation in order to hold more power in controlling the peoples of their territory, among other reasons.
The most direct and obvious cause of the Crusades is the Muslim hostility evident in their takeover of the Holy Land and their battling with Spain. The religious significance of the area was too great for the Christians to simply ignore the fact that Muslims had control over a sacred land. Also noteworthy to consider is the massive population increase that began in the tenth and eleventh centuries primarily due to the use of a mechanized water wheel for grinding grain and a good climate for the time period. Europe is a very tiny area, so it was necessary to migrate into new areas so that people had enough land to support themselves and their family. The most important cause of the crusades, though, was the reforms that occurred in the Roman Catholic Church in the eleventh century. After Charlemagne's death, widespread corruption occurred in the church in the form of lay investiture and simony. During the eleventh century, a series of moral and political reforms were taken in the Roman Catholic Church to prevent these acts from being committed, and also to strengthen the waning rule of the Roman Catholic Church. So, it should be noted that this was a public display for the Church to prove themselves to their public. The purpose of going to the Crusades was to prove that the reforms were successful in reshaping the Church and that they had the abilities and strength to perform a huge military task. After all, if they could conquer the Muslims in Jerusalem, why couldn't they rule the land in Europe efficiently?.