"We would also say this to those who are alien to our faith and ask us to fight for the community and to kill men: that is is also your opinion that the priests of certain images and wardens of the temples of the gods, as you think them to be, should keep their right hand undefiled for the sake of sacrifices, that they may offer the customary sacrifices to those who you say are gods with hands unstained by blood and pure from murders. And in fact when war comes you do not enlist the priests. If, then, this is reasonable, how much more reasonable is it that, while others fight, Christians also should be fighting as priests and worshippers of God, keeping their right hands pure and by their prayers to God striving for those who fight in a righteous cause and for the emperor who reigns righteously, in order that everything which is opposed and hostile to those who act rightly may be destroyed? Moreover, we who by our prayers destroy all demons which stir up wars, violate oaths, and disturb the peace are of more help to the emperors than those who seem to be doing the fighting. We who offer prayers with righteousness, together with ascetic practices and exercises which teach us to despise pleasures and not to be led by them, are cooperating in the tasks of the community. Even more do we fight on behalf of the emperor. And though we do not become fellow-soldiers with him, even if he presses for this, yet we are fighting for him and are composing a special army of piety through our intercessions to God."
This is a remarkable argument for tolerance towards Christians, but it is one that requires a government that believes in prayer and a god who answer prayers. Rome honored the gods and had civil religion with priests who offered sacrifices and prayers on behalf of the emperor and his people. It is interesting to think about how our civil religion in America functions: who are our priests? what do we sacrifice? where are our temples? But this is not the direction I want to take. Origen's argument is profound, even if our secular age does not have the ability within itself to acknowledge the power of his argument since it requires the belief in prayer that works. If you want the help of the Christian God, and if the help of the Christian God comes through prayer and piety, then you must have Christians who are pure and holy in order for their prayers to be heard. The Romans believed that the gods answer the prayers of the righteous. Asking Christians to violate their laws by requiring them to fight Rome's wars and then expecting them to also pray for divine favor is like asking a good Roman to offer a sacrifice in the wrong way. Roman sacrifices required that the supplicant perform the proper rituals in their necessary form and order more than that the supplicant maintain personal sincerity. In other words, the sincerity and righteousness of a person was measured more by their respect of the rituals that honored the gods than that they approach the gods with "a humble and contrite heart." Christians must be enabled to be Christians, which Origen assumed included nonviolence based on the teachings of Jesus, in order for them to be "useful" to the government.
This is not an argument in favor of just war theory, but it does affirm the Christian's responsibility to pray that the government take up a "righteous cause." We pray for "those who fight in a righteous cause and for the emperor who reigns righteously." Origen's wording is subtle because he knows that we don't blindly pray for our governments and their success, for it was true then as it is now that not every cause is righteous and neither is every emperor. We pray that when our governments act, they would do so justly, knowing full well that so often they do not. Origen's point is that Christians qua Christians do not fight wars, regardless if they are just or not. As he said before the excerpt above, Christians arm themselves with the armor of God, and "the more pious a man is, the more effective he is in helping the emperors - more so than the soldiers who go out into the lines and kill all the enemy troops they can." Can we even imagine what would happen if all the Christians in the world refused to fight in wars and gathered to pray? Not being able to do so is, in my opinion, a lack of eschatological imagination that the NT and much (not all) of the Christian tradition requires.