Military Orders
The Dilemma of the Christian Soldier
During the centuries when the Church was a persecuted sect within the Roman Empire, it was a church of pacifists. The essentially pacific message of the New Testament was powerfully reinforced by the tradition of non-resistance established by generations of martyrs. The whole tenor of the early Church was one of withdrawal from the matters of the world, and making war was certainly of this world.
When Christianity became a favored religion in the 4th century, the situation changed dramatically. Many people converted and some of those converts were soldiers. Once Theodosius made Christianity the official religion of the Empire, this meant that indeed the majority of the soldiers were Christian.
As in so much else, the writings of Augustine of Hippo were crucial in expounding and justifying the position accepted later by the medieval Church. After considering all the arguments, Augustine developed the theory of the just war. Simply put, he argued that certain types of war were clearly justified—such as when attacked unprovoked. Remember that Augustine lived to see Rome sacked and to see his own city of Hippo (in North Africa) undergo a terrible siege by Germanic invaders. God did not want his Church to perish, Augustine argued, and there were plenty of examples from the Old Testament to support the position. The killing that a Christian did in a war was still a sin, for which penance was necessary, but war was recognized by the Church as an unavoidable evil in an evil world.
Peace of God and Truce of God
Armed pilgrimage
Knights of St. Peter
The Augustinian position became widely accepted , with later generations elaborating on exactly what constituted a just war. Fighting to defend the Church itself was, of course, the most justifiable form of war. This was exactly the tradition on which Pope Urban drew when he called upon the Franks to liberate the Holy Sepulcher in 1095. But Urban called upon the Frankish nobility to do the fighting, of course; it never occurred to him to call upon monks, for monks did not fight.
As it happened, it was not monks who decided to fight, but rather it was knights who decided to live as monks. This transformation was entirely spontaneous and came from the knights themselves.