In the next place, Celsus urges us “to help the king with all our might, and to labour with him in the maintenance of justice, to fight for him; and if he requires it, to fight under him, or lead an army along with him.” To this our answer is, that we do, when occasion requires, give help to kings, and that, so to say, a divine help, “putting on the whole armour of God.” And this we do in obedience to the injunction of the apostle, “I exhort, therefore, that first of all, supplications, prayers, intercessions, and giving of thanks, be made for all men; for kings, and for all that are in authority;” and the more any one excels in piety, the more effective help does he render to kings, even more than is given by soldiers, who go forth to fight and slay as many of the enemy as they can.
And to those enemies of our faith who require us to bear arms for the commonwealth, and to slay men, we can reply: “Do not those who are priests at certain shrines, and those who attend on certain gods, as you account them, keep their hands free from blood, that they may with hands unstained and free from human blood offer the appointed sacrifices to your gods; and even when war is upon you, you never enlist the priests in the army. If that, then, is a laudable custom, how much more so, that while others are engaged in battle, these too should engage as the priests and ministers of God, keeping their hands pure, and wrestling in prayers to God on behalf of those who are fighting in a righteous cause, and for the king who reigns righteously, that whatever is opposed to those who act righteously may be destroyed!”
And as we by our prayers vanquish all demons who stir up war, and lead to the violation of oaths, and disturb the peace, we in this way are much more helpful to the kings than those who go into the field to fight for them. And we do take our part in public affairs, when along with righteous prayers we join self-denying exercises and meditations, which teach us to despise pleasures, and not to be led away by them. And none fight better for the king than we do. We do not indeed fight under him, although he require it; but we fight on his behalf, forming a special army— an army of piety— by offering our prayers to God.
Source: Against Celsus (New Advent)