Knightly Piety refers to a specific strand of Christian belief espoused by knights during the Middle Ages. The term comes from Ritterfrömmigkeit coined by Adolf Waas in his book Geschichte der Kreuzzüge. Many scholars debate the importance of knightly piety, however it is apparent as an important part of the chivalric ethos based on its appearance within the Geoffroi de Charny's Book of Chivalry as well as much of the popular literature of the time.
A relationship between Christ and warrior is first seen in secular sources dating back to Carolingian times. This is evident within the chansons de geste or songs of heroic deeds. Both the Chanson de Roland and the Chanson de Guillaume demonstrate Christian themes in their tales of the fight against the nonbeliever. Both have elements of an earthly as well as a spiritual fight. Thus by the time of the Chivalric Codes Christianity is already firmly entrenched within the warrior classes. Keen dedicates much of the credit to the effective teaching of the priesthood as well as the close relationship between the nobility and the monasteries.
However, the Catholic Church traditionally had an uneasy relationship with secular warriors dating back to the time of the Roman Empire. It was generally accepted by the Church that warfare and killing was sinful. During the later period of the Empire, theologian Augustine of Hippo wrote of a Just War in the City of God. In this theory he claimed it would be sinful not to defend God if there was no other way to resolve a conflict.
During the late tenth and early eleventh centuries the Church involved itself more with warfare. First came the Peace and Truce of God movements. This was one way for the Church to attempt to Christianize society while at the same time protect non-combatants. The movement promised severe punishments to those who broke this law. However, it was met with mixed success.
In 1095, Pope Urban II preached the First Crusade at Clermont. Here, the Church officially sanctioned lay knights fighting for the Faith when Urban said that any who fought would be absolved of their sins rather than tarnish their soul for killing. By this time knights were already concerned with their immortal soul enough to fight for the Church. By the time the Church began to accept warfare and create the idea of a holy war, piety had already become entrenched in the warfare of the lay knight. However, as the time of increasing church involvement was the formative period of the Chivalric Codes, it helped add another dynamic to the Ritterfrömmigkeit.