History of Christianity
The Holy Land had been part of the Roman Empire, and thus Byzantine Empire, until the Islamic conquests of the seventh and eighth centuries.  Thereafter, Christians had generally been permitted to visit the sacred places in the Holy Land until 1071, when the Seljuk Turks closed Christian pilgrimages and assailed the Byzantines, defeating them at the Battle of Manzikert.  Emperor Alexius I asked for aid from Pope Urban II (1088-1099) for help against Islamic aggression.  Instead of sending money, Urban II called upon the knights of Christendom in a speech made at the Council of Clermont on 27 November 1095, combining the idea of pilgrimage to the Holy Land with that of waging a holy war against infidels.