Originating from the institutions of absolution and commutation of the penitential in the early Middle Ages, indulgence was the outcome of private penance, and closely related to the theories of "Merit and Treasury" and "Purgatory" in the high Middle Ages. The indulgences in fact enhanced the social involvement and secularization of the Catholic church in the high and late Middle Ages. The collected funds supported many developments of the church and society at that time. To understand the indulgence critically will be helpful for us to understand the cultural meaning of the medieval social life and eliminate the misunderstandings in modem thoughts and cultures.