In the eleventh century the papacy had strong reasons for wanting to launch an expedition against Muslims in the East. If the pope could muster a large army against the enemies of Christianity, his claim to be the leader of Christian society in the West would be strengthened. Moreover, in 1054 a serious theological disagreement had split the Greek Church of Byzantium and the Roman Church of the West. The pope believed that a crusade would lead to strong Roman influence in Greek territories and eventually the reunion of the two churches.

Popes and other church officials gained support for war in defense of Christianity by promising spiritual benefits to those who joined a campaign or died fighting. Church leaders said that these people would be forgiven for their sins without having to do penance, that is, without having to confess to a priest and carry out some action to make up for the sins. Preachers communicated these ideas widely and told stories about warrior-saints who slew hundreds of enemies.

Religious zeal led increasing numbers of people to go on pilgrimages to holy places, including Jerusalem. The Arab Muslims who had ruled Jerusalem and the surrounding territory for centuries allowed Christian pilgrims to travel freely, but in the late eleventh century the Seljuk Turks took over Palestine, defeating both Arab and Byzantine armies, and pilgrimage became more difficult. The Byzantine emperor at Constantinople appealed to western European Christians for support. The emperor’s appeal fit well with papal aims, and in 1095 Pope Urban II called for a great Christian holy war against the infidels — a term Christians and Muslims both used to describe the other. Urban urged Christian knights who had been fighting one another to direct their energies against those he claimed were the true enemies of God, the Muslims.