Pope Urban II's Sermon at the Council of Clermont Motivates Crusaders to Conquer Jerusalem and Form the Kingdom of Jerusalem

In March 1095 Byzantine emperor Alexios I Komnenos (Alexius I Comnenus, Ἀλέξιος Α' Κομνηνός) sent his ambassador to the Pope, asking for help defending his empire against the Muslim Seljuk Turks. Responding to the emperor's request, in November of that year Pope Urban II delivered a sermon at the Council of Clermont that was characterized as "the most effective single speech in European history." He motivated the attending nobility and the people to wrestle the Holy Land from the hands of the Seljuk Turks. One the tools he used to motivate was his declaration that he remitted all penance incurred by who had confessed their sins in the Sacrament of Penance, considering participation in the crusade equivalent to a complete penance. This is the earliest record of a plenary indulgence.

Urban II's speech led to the First Crusade. Crusader armies marched on Jerusalem, sacking several cities on their way. In 1099 they took Jerusalem and massacred the population. As a result of the First Crusade, and the enormous destruction involved, several small Crusader states were created, notably the Kingdom of Jerusalem.