Christianity overcame pagan Rome by nonviolence.
But when Christianity became the religion of the Empire, then the stoic and political virtues of the Empire began to supplant the original theological virtues of the first Christians. The heroism of the soldier supplanted the heroism of the martyr—though there was still a consecrated minority, the monks, who kept the ideal of charity and martyrdom in first place.
The ideal of self-sacrifice was never altogether set aside—on the contrary! But it was transferred to a new sphere. Now the supreme sacrifice was to die fighting under the Christian emperor. The supreme self-immolation was to fall in battle under the standard of the Cross. In the twelfth century even monks took up the sword, and consummated their sacrifice of obedience by dying in battle against infidels, against heretics.
Unfortunately, they also fought other monks, and this was not necessarily regarded as virtue. But it does show what comes of living by the sword!
Christian chivalry was the fruit of a union between Christian faith and Roman, Frankish, or Germanic valor. In other words, Christians did here what they also did elsewhere: they adopted certain non-Christian values and “baptized” them, consecrating them to God. Christianity might just as well have turned to the East and “baptized” the nonmilitant, contemplative, detached, and hieratic institutions of the Orient. But by the time Christianity was ready to meet Asia and the New World, the Cross and the sword were so identified with one another that the sword itself was a cross. It was the only kind of cross some conquistadores understood.
There was no further thought of Christianizing the ideals and institutions of these ancient civilizations: only of destroying them, and bringing their people into subjection to the militant Christianity of Europe. Hence the strange paradox that certain spiritual and largely nonviolent ideologies which were in fact quite close to the Gospel were attacked and coerced in the name of Christ by the Christian soldier who was often no longer a Christian except in name: for he was violent, greedy, self-complacent, and supremely contemptuous of anything that was not a perfect reflection of himself.
(From Conjectures of a Guilty Bystander by Thomas Merton, pg. 101, 1966, Doubleday)