The First Crusade was called in November 1095 by Pope Urban II at the town of Clermont in central France. The pope made a proposal: 'Whoever for devotion alone, but not to gain honour or money, goes to Jerusalem to liberate the Church of God can substitute this journey for all penance.' This appeal was the combination of a number of contemporary trends along with the inspiration of Urban himself, who added particular innovations to the mix. For several decades Christians had been pushing back at Muslim lands on the edge of Europe, in the Iberian peninsula, for example, as well as in Sicily. In some instances the Church had become involved in these events through the offer of limited spiritual rewards for participants.
Urban was responsible for the spiritual well-being of his flock and the crusade presented an opportunity for the sinful knights of western Europe to cease their endless in-fighting and exploitation of the weak (lay people and churchmen alike) and to make good their violent lives. Urban saw the campaign as a chance for knights to direct their energies towards what was seen as a spiritually meritorious act, namely the recovery of the holy city of Jerusalem from Islam (the Muslims had taken Jerusalem in 637). In return for this they would, in effect, be forgiven those sins they had confessed. This, in turn, would save them from the prospect of eternal damnation in the fires of Hell, a fate repeatedly emphasised by the Church as the consequence of a sinful life.
Within an age of such intense religiosity the city of Jerusalem, as the place where Christ lived, walked and died, held a central role. When the aim of liberating Jerusalem was coupled to lurid (probably exaggerated) stories of the maltreatment of both the Levant's native Christians and western pilgrims, the desire for vengeance, along with the opportunity for spiritual advancement, formed a hugely potent combination. Urban would be looking after his flock and improving the spiritual condition of western Europe, too. The fact that the papacy was engaged in a mighty struggle with the German emperor, Henry IV (the Investiture Controversy), and that calling the crusade would enhance the pope's standing was an opportunity too good for Urban to miss.
A spark to this dry tinder came from another Christian force: the Byzantine Empire. Emperor Alexios I feared the advance of the Seljuk Turks towards his capital city of Constantinople. The Byzantines were Greek Orthodox Christians but, since 1054, had been in a state of schism with the Catholic Church. The launch of the crusade presented Urban with a chance to move closer to the Orthodox and to heal the rift.