The indulgences received a new form, however, and became a part of the regular Church administration, when the popes discovered the possibilities which lay in this institution for the advancement of their own power and the furtherance of their own interests. This discovery seems to date from the time of the Crusades. The crusading-indulgences, granted at first only to those who actually went to the Holy War, subsequently to those also who contributed to the expense of the expedition, were virtually the acceptance of this work as a substitute for any penance which the Church might otherwise require. As zeal for the Crusades began to wane, the indulgences were used more and more freely to stimulate lagging interest; their number was greatly increased, and those who purchased the indulgences with money far outnumbered those who actually took the Cross. Failing in their purpose as an incentive to enlistment in the crusading armies, they showed their value as a source of income, and from the beginning of the XIV. Century the sale of indulgences became a regular business. About the same time a new kind of indulgence arose to take the place of the now somewhat antiquated crusading-indulgence. This was the Jubilee-indulgence, and had its origin in the Jubilee of 1300. By the Bull Antiquorum Habet Fide, Boniface VIII. granted to all who would visit the shrines of the Apostles in Rome during the year 1300 and during each succeeding centennial year, a plenary indulgence.[14] Little by little it became the custom to increase the number of these Jubilee-indulgences. Once in a hundred years was not often enough for Christians to have a chance for plenary forgiveness, and at last, unwilling to deprive of the privileges of the Jubilee those who were kept away from Rome, the popes came to grant the same plenary indulgence to all who would make certain contributions to the papal treasury.[15]