"In an ideological sense, the growth of the military orders was fostered by a theological shift in attitude of the Church towards just warfare. Prior to the First Crusade, the Peace of God movement had developed a policy within the Church that knights should be restricted from fighting among themselves and instead put their energies into fighting for the Church. Prior to this, the knightly class had been in almost a constant state of civil war amongst themselves and with the Church.
At the Council of Clermont in 1095, Pope Urban II advocated for the service of secular knights for the church by declaring that “you have promised more firmly than ever to keep the peace among yourselves and to preserve the rights of the church, there remains still an important work for you to do.” By deliberately trying to encourage secular knights to fight for God in the Crusades, the Church created an acceptable form of warfare within Christian ideology.
While Urban was not encouraging monks to go on Crusade, the military orders could not have existed without the ideology behind his speech because of Christianity’s traditional pacifism. The key element to this new ideology was the idea of miles Christi. The term miles Christi was essential to the ideology of the military orders and is critical in understanding their rise to power. Coming into use through the writings of St Augustine and literally meaning “soldier of Christ,” it reflects the duty of a Christian knight to protect Christendom from outside evils. As the monk fought the spiritual enemies of the Church from within his monastery, the soldier of Christ fought Christianity’s physical enemies. Instead of committing the sin of murder, the traditional Christian view of warfare, a miles Christi was actually doing penance for his sins.
The Hospitallers and the Templars fought Islam in the Holy Land and the Teutonic Knight was hugely instrumental in fighting paganism in eastern Europe. The knights of the military orders were by definition miles Christi and their development and rise to power would not have been possible without the widespread acceptance of this idea.
To the Church, the military orders provided the perfect outlet for the knightly class even when there was no official crusade being promoted. Therefore, the Church was more than willing to support them and allow their power to grow both in the Holy Land and in Europe. As the dominant European ideology, the Church’s changing theology played a large role in the growth of the military orders, and was aided most directly by the theological support of St. Bernard."
Illustration Templar and Hospitaller Knights, source; This blog quotes form a publication based on a 2014 paper by Sarah E. Hayes in the Gettysburg Historical Journal, www.medievalists.net