Pope Urban II launched the First Crusade in 1095 in order to rid the Holy Land of Seljuk Turk influence and claim it for Christendom.  This was considered quite important in the Catholic Church; in order for Christ to return, Christians had to possess the Holy Land.  By defeating the armies of the "infidel," Christendom could demonstrate to God that it was willing to advance His kingdom on Earth.  Pope Urban II knew that this was an arduous journey and that it would take a great deal to get kings and nobility to quite fighting among themselves, so he promised salvation to those who "took up the Cross" and fought in the Crusades.  The message was initially meant for knights and nobility, but many from the lower classes went to fight in the Middle East in search of God's favor and salvation.  If one died fighting the Muslims, one was supposed to gain immediate entry to Heaven.  This was the primary motivation for fighting in this Crusade; for later Crusades, popes had to promise material gain, as people were well aware of the dangers.  

The undertaking of a journey such as Pope Urban II was suggesting was a major, life-changing and life-threatening proposition. He needed to be persuasive and to offer the possibility of great reward for those willing to undertake this potentially very dangerous adventure. The journey would be long and difficult; the enemy would be skilled in fighting and unmerciful to any captives. In an age when the Roman Catholic Church was powerful in the secular as well as religious world, Pope Urban II could inspire kings and knights as well as commoners by promising that those who died while traveling to or fighting in the Holy Land as part of the Crusades against the infidel Muslims would be forgiven all their sins.