This is where indulgences came in, particularly in connection with armed, fighting pilgrimages, or as we call them, Crusades. To try to drum up troops to free the Holy Land, the Pope had promised certain unspecified spiritual benefits to all who fought for the faith; the Church then needed to figure out exactly what these benefits were. The doctrines outlined above were elaborated to a large extent to answer this question. The Church eventually decided that going on a Crusade would remit the entire temporal penalty due to sins that had been confessed to a priest and absolved. But since not everyone could go on Crusade—women, the sick, the aged, etc.—and not everyone who wanted to go on Crusade could afford it, it was decided that if you paid the way for someone else to go on Crusade, you would receive the benefits of Crusading yourself. These benefits would be paid to you out of the Treasury of Merits, the collection of good works performed by Christ and the saints above and beyond what was necessary for their own salvation. The Pope was the custodian of this Treasury and could call a Crusade and authorize a withdrawal for the participants. (Not all the Crusades were to the Holy Land: the Reconquistà which drove the Muslims out of Spain actually began before the Crusades proper, and other Crusades were called against the Albigensians in France, pagans in the Baltic, the Hussites in Bohemia, and even the Pope's political rivals.) With the end of the Crusades, the practice of selling indulgences, as these crusading benefits were called, continued. Some were limited indulgences, which paid only for a specific period of time, say, 10 years of penance. Others were plenary indulgences, which paid the whole penalty. Leo's indulgence sale was the latter type.
The Indulgence Controversy and the 95 Theses
First, following standard procedures, Luther sent a copy of the theses to his spiritual superior, who happened to be Archbishop Albrecht of Mainz. Like Queen Victoria, he was not amused. Second, some of Luther's students got hold of the theses, thought, "this is really hot stuff," and sent them off to a printer AFTER TRANSLATING THEM INTO GERMAN. Much to everyone's surprise, the 95 Theses became a runaway best seller, with translations following into most European languages. Despite the best efforts of professors since then, this was the only time in history that an academic exercise has generated such an incredible volume of sales. Luther thus unexpectedly found himself at the center of an international controversy over indulgences.