What defined the crusades in contrast to other wars was the opportunity for warriors to win a spiritual reward, the indulgence. In The Crusade Indulgence. Spiritual Rewards and the Theology of the Crusades, c. 1095-1216 Ane L. Bysted examines the theological and institutional development of the indulgence from the proclamation of the First Crusade to Pope Innocent III.
This first comprehensive study of crusade indulgences in more than a hundred years challenges some earlier interpretations and demonstrates how theologians, popes, and crusade preachers in the 12th century formed the concept of indulgences and argued that fighting for Christ and the Church was meritorious in the sight of God and thus worthy of a spiritual reward proclaimed by the Church