Aineo should read "The Decline and Fall of the Roman Empire" by the English historian Edward Gibbon.
The Holy Land was under the jurisdiction of moderate Arabs for four hundred years, from the conquest of Jerusalem under the Caliph Umar ibn al-Khattab (A.D. 637) to the conquest of the Holy Land by the Seljuk Turks in the mid-eleventh century. During this time, the Muslim rulers permitted Christian pilgrims full rights to visit the holy sites, including the churches in Jerusalem and Bethlehem. After the conquest by the Turks, the pilgrims were less tolerated, and may also have been mistreated; historians are really not sure. But the supposed "need" to reclaim the Holy Land from the "infidel" Turks was the obstensible purpose for the First Crusade. Pope Urban II commissioned it (1095) in his famous speech in Clermont, France.
The "real" motives for the Crusades, other than the "camouflage" reason, have long been debated. The pope wished to reassert his authority over the secular Christian rulers, including the Holy Roman emperor, since the eleventh century had witnessed the struggle between pope and emperor for supermacy in the West. The schism between the Western Church and the Eastern (Orthodox) Church had also occurred about 40 years earlier, and this fact may have contributed to the motive for the Crusade. Regardless, the Eastern Roman (Byzantine) Empire was humbled by the sacking of Constantinople in 1204 during the Fourth Crusade.
There may have been another demographic reason for the Crusades. The population of Europe rose considerably during the eleventh century, thanks to a moderating climate and to agricultural innovations. Like any society with a disproportionate number of young people, the European society of c. 1100 had "surplus" human energy, which was diverted to the Holy Land rather than inside Europe itself.