Nonviolence
If you look at texts in the hundred years preceding 313, it was unthinkable that a Christian would fight in the army. The army was killing Christians; Christians were being persecuted. By the year 400, the entire army had become Christian, and Christians were killing the “pagans.” In a two-hundred-year period, we went from being complete outsiders to directing the inside! Once you are inside, you have to defend your power and your privilege.
It is during this transition that people like St. Anthony of the Desert, John Cassian, Evagrius Ponticus, and the early monks went off to Egypt, Syria, and the deserts of Palestine. They critiqued the self-protective, privileged lifestyle of mainline Christianity by utterly leaving it! Soon they learned and taught a different way of seeing called “contemplation.” From that point through the modern period, most governments assumed that Christian monks and priests could not, or should not, wage war or kill others.
Why this split between two brands of Christianity? Why were some expected to take the Sermon on the Mount seriously, while the rest were exempt? Even as recently as the Vietnam War, laity could kill, while the clergy could not. As a Franciscan, I received an immediate draft deferment in the 1960s. When the Gospel is heard and understood at its deepest level, Christians cannot and will not kill or wage war.