Answer:
The Catholic Church asked people to help their fellow Christians, those who were still alive and under persecution by the Muslim forces in the Holy Land, an plenary indulgence if they went on the Crusade to extend mercy on their fellow Christians.
What most people do not understand is that an indulgence had nothing to do with forgiveness of sins. Indulgences have been part of the Christian Church since the very early centuries when people asked others who were about to be killed by the Romans for their prayers.
Since then the theology of indulgences has not changed at all, although due to the unfortunate misunderstanding of Martin Luther, indulgences have been maligned and misunderstood since that time.
An indulgence can ONLY be gained by someone who is in a state of grace - all their sins forgiven already. NO ONE in a state of sin can earn an indulgence - not possible. An indulgence is a prayer of the Church or the saints for the remission of temporal punishment due for already forgiven sin. Thus it would be time off in purgatory, no one in a state of sin could earn an indulgence nor “earn” heaven - such an idea is atrocious and heresy.
