God, the all-wise and all-powerful, is in charge of the world. We are not in charge of the course of events, responsible (as in most settings we are not able) to prevent atrocities or vindicate justice. WE need not defend ourselves; God has always protected his own, and will protect us in the future if that is his will. If it is not his will, our mobilizing for our own defense may be against his providential purpose. He may want to chastise us for our sins. Or He may want to use our suffering to 'sanctify his name,' that is, through martyrdom, the opposite of chastisement. If evil has its way, it is under God's permission and will not last. When he does triumph it will not be our doing and will not depend on our providing His troops. This is the explicit instruction of Rom. 12:19, leaving vengeance to God. This vision undercuts without needing to say so the consequentialism that is indispensable to the case for war in the name of social 'responsibility.'
Were Christians before Constantine pacifist? Certainly not, if we give the term an ahistorically modern definition. They did not advocate arms reduction negotiations nor an alternate world order that would do away with the occasion for war. The pax Romana in fact claimed already to be that. They were not consulted by Caesar about how to run the empire, since neither he nor they knew about 'the consent of the governed.' Thus, they neither asked Caesar to implement the non-violent Christian ethic from his throne nor measured the Christian ethic consequentially by whether it could be suited to run an empire. They did not refuse to serve when subject to universal conscription, since there was none. They accepted non-lethal work in the service of the peacetime military bureaucracy. Their clash with the military establishment was no rooted only in abhorrence of killing. Nor was it limited only to their abhorrence of idolatry. It was rooted in a fundamentally anti-tyrannical and anti-provincial vision of who God is and of God's saving purposes in the world.
Augustine's argument [for just war] is negative legalism, not a clear imperative. War cannot be forbidden, he argues, because John the Baptist did not forbid it, Jesus did not scold the centurion, Peter did not tell Cornelius to resign, God may have providentially subjected you to an ungodly king, Christian emperors have conquered pagan nations, and the world is miserable anyway. There is Augustine never a joyful Gospel confidence that bloodshed pleases or praises God...Augustine's mood was a 'mournful' pastoral adjustment to a world of which we cannot in any case ask that God's will be done. What has changed is not one ruling on what God's will is, but the entire setting in which doing God's will can be thought about. The Neoplatonic grid, according to which God's will cannot really be done, and the sociology of the imperial Church, according to which 'Christian' means everybody, have defined a whole different world.