Christians generally think of Jesus as a man of peace, though he is reported to have said once that he had come “not to bring peace but a sword.” For their first three centuries, Christians as a group were largely on the receiving end of institutionalized violence in the form of Roman persecution, but that situation changed with the reign of Emperor Constantine. Saint Ambrose (c. 340-397), Saint Augustine’s mentor, was one of the first to fashion a systematic philosophy of war based on the Old Testament. He interpreted war as an instrument of divine judgment on God’s enemies, but insisted that Christians must interpret relevant texts of the New Testament in an inward or spiritual sense and seek peace in all their personal relationships.

Augustine (354-430) further developed the theme of institutional dimensions of violent means in his extensive reflections on criteria for a “just war.” Religiously legitimate warfare must first be based on a just cause such as military aggression or violation of fair commerce. It must, secondly, be seen as a last resort after all peaceful means have been exhausted. Third, only a government with competent authority can declare war, and only, fourthly, with a realistic probability of success. Fifth, violence must be proportionate to the good results envisioned, and finally, combatants must have proper intention and may not target civilians directly.

Christians have continued to refine Augustine’s theory over the centuries with very mixed results. Some would argue that the Crusades did not fulfill the criteria for a just war in Augustine’s terms, but were closer to the use of secular power to suppress heresy. And in the “age of exploration,” Christian armies engaged in violence on a large scale in many parts of the world, and by no means always in defense. In 1452, Pope Nicholas V charged the King of Portugal as follows: “In the name of our apostolic authority, we grant to you the full and entire faculty of invading, conquering, expelling and reigning over all the kingdoms, the duchies of the Saracens, of pagans and of all infidels, wherever they may be found; of reducing their inhabitants to perpetual slavery, of appropriating to yourself those kingdoms and all their possessions, for your own use and that of your successors.” More recently, Christians have continued to debate the religious justification of violent means. Cardinal Ratzinger (who later became Pope Benedict XVI in 2005), for example, argued that the U.S. invasion of Iraq did not qualify as a just war, but his view has been widely disputed even among Catholics.