What I’m trying to do here is determine how Christianity developed its views on violence in general, and on war in particular.
The Ante-Nicene Fathers (prior to Constantine and the Council of Nicaea) were unanimous or close to it in taking a pacifist position. Christians were not to kill. Pure and simple. Many argued that Christians were not even allowed to kill in self-defense. This was based on the way Jesus allowed himself to be arrested without a fight, on the belief that this world and its material possessions were unimportant (and thus should not be defended at the cost of someone’s life), and combined with that, a focus on the life after death, meaning that one’s salvation was more important than one’s life. Christians were not allowed to join the Roman army either, both because they would have to kill and because the army was pagan and held pagan ceremonies.
But by the third century, Christians were serving in the army, and then Constantine accepted Christianity in 312. Now the Roman empire was ruled by a Christian who was tasked with defending it by force of arms. Christian thinking changed rather quickly; by the end of the fourth century, Ambrose and Augustine had laid down the foundations of “just war” by which Christians not only could kill, but were commanded to kill when ordered to do so by the authorities.
This explains how Christians could defend the empire. But it also leads, gradually, to the crusades and to the militarized Vatican of the medieval popes.
The question I am working on now is the development of the lawful use of violence against heresy and heretics. After rather painful research (there doesn’t seem to be much in this area), I have come to the conclusion that while heresy was quickly made into a capital crime (the Codex Theodosianus, etc.), there were very few, perhaps no, executions after 400 until the middle of the eleventh century. I think this is because secular authority, responsible for enforcing these laws, was becoming more and more fragmented, and the Church was still concentrating on converting the rest of Europe, a task it achieved before 1100.
Now, with the rise of the militant popes and especially with Innocent III, heresy became a central concern, and this time the Church, having become accustomed to using violence against the Muslims, began to turn it against Christians who had independent ideas about religion. Innocent did not create the Inquisition, but he gave it a standing and powers that it had never had before. He also instigated the Albigensian Crusade, which had other reasons beside religion (land grabs, mainly), but which was justified on the grounds that the Cathar heresy (basically, a form of dualism with elements of the Docetic heresy of the first century) needed to be stamped out. Innocent was, perhaps above all else, consumed with the desire to crush all forms of dogma other than his own.
What drove him to this point, and how he was able to bring Europe along with him, are points I am still working out. One element of this may be insecurity; a secure empire, a secure church, is more likely to tolerate and even welcome divergent views. But Constantine and Theodosius ruled an empire in big trouble and could not afford to allow any doctrines that might interfere with their need for unity. What, I have to ask, was Innocent’s fear? He was the most powerful pope in history, at the peak of the Vatican’s power.
To be continued.