And of these sayings the teaching is this: Bless those who curse you, and pray for your enemies, and fast for those who persecute you. For what reward is there for loving those who love you? Do not the Gentiles do the same? But love those who hate you, and you shall not have an enemy. Abstain from fleshly and worldly lusts. If someone strikes your right cheek, turn to him the other also, and you shall be perfect.
Didache (A.D. 80-140), Chapter 1.

Peter answered: Why are you afraid of hearing frequently of peace? For do you not know that peace is the perfection of law? For wars and disputes spring from sins; and where there is no sin, there is peace of soul; but where there is peace, truth is found in disputations, righteousness in works. – Peter, Recognitions of Clement, book 2, chapter 36

We who hated and destroyed one another, and on account of their different manners would not live with men of a different tribe, now, since the coming of Christ, live familiarly with them, and pray for our enemies, and endeavour to persuade those who hate us unjustly to live conformably to the good precepts of Christ, to the end that they may become partakers with us of the same joyful hope of a reward from God the ruler of all.
Justin Martyr (A.D. 160), Ante-Nicene Church Fathers – Vol. 1, Page 167.

We who formerly used to murder one another do not only now refrain from making war upon our enemies, but also, that we may not lie nor deceive our examiners, willingly die confessing Christ.
Justin Martyr (A.D. 160), Ante-Nicene Church Fathers – Vol. 1, Page 176.

“I have no desire to rule. I crave not riches. I decline military command.” – Tatian, Adversus Graecos oratio, Chapter 11 (PG, VI, 829)

(The following was written by a pagan Roman emperor regarding his experience with professing Christian soldiers in his army who refused to fight but instead offered prayers.) The Emperor Caesar Marcus Aurelius, to the People of Rome, and to the sacred Senate… I was surrounded by the enemy; And the enemy being at hand… there was close on us a mass of a mixed multitude of 977,000 men, which indeed we saw… Having then examined my own position, and my host, with respect to… the enemy, I quickly betook myself to prayer to the gods of my country. But being disregarded by them, I summoned those who among us go by the name of Christians. And having made inquiry, I discovered a great number and vast host of them, and raged against them, which was by no means becoming; for afterwards I learned their power. Wherefore they began the battle, not by preparing weapons, nor arms, nor bugles; for such preparation is hateful to them, on account of the God they bear about in their conscience. Therefore it is probable that those whom we suppose to be atheists, have God as their ruling power entrenched in their conscience. For having cast themselves on the ground, they prayed not only for me, but also for the whole army as it stood, that they might be delivered from the present thirst and famine. For during five days we had got no water, because there was none; for we were in the heart of Germany, and in the enemy’s territory. And simultaneously with their casting themselves on the ground, and praying to God (a God of whom I am ignorant), water poured from heaven, upon us most refreshingly cool, but upon the enemies of Rome a withering hail. And immediately we recognized the presence of God following on the prayer – a God unconquerable and indestructible. Founding upon this, then, let us pardon such as are Christians, lest they pray for and obtain such a weapon against ourselves. And I counsel that no such person be accused on the ground of his being a Christian. But if any one be found laying to the charge of a Christian that he is a Christian, I desire that it be made manifest that he who is accused as a Christian, and acknowledges that he is one, is accused of nothing else than only this, that he is a Christian; but that he who arraigns him be burned alive. And I further desire, that he who is entrusted with the government of the province shall not compel the Christian, who confesses and certifies such a matter, to retract; neither shall he commit him. And I desire that these things be confirmed by a decree of the Senate. And I command this my edict to be published in the Forum of Trajan, in order that it may be read. The prefect Vitrasius Pollio will see that it be transmitted to all the provinces round about, and that no one who wishes to make use of or to possess it be hindered from obtaining a copy from the document I now publish.
Emperor Caesar Marcus Aurelius (A.D.160), Ante-Nicene Church Fathers – Vol. 1, Page 187.

We who were filled with war, and mutual slaughter, and every wickedness, have each through the whole earth changed our warlike weapons – our swords into ploughshares, and our spears into implements of tillage – and we cultivate piety, righteousness, philanthropy, faith, and hope, which we have from the Father Himself through Him who was crucified.
Justin Martyr (A.D. 160), Ante-Nicene Church Fathers – Vol. 1, Page 254.

No new covenant was given, but they used the Mosaic law until the coming of the Lord; but from the Lord’s advent, the new covenant which brings back peace, and the law which gives life, has gone forth over the whole earth, as the prophets said: “For out of Zion shall go forth the law, and the word of the Lord from Jerusalem; and He shall rebuke many people; and they shall break down their swords into ploughshares, and their spears into pruning-hooks, and they shall no longer learn to fight.”… the law of liberty… caused such a change in the state of things, that these [nations] did form the swords and war-lances into ploughshares, and changed them into pruning-hooks for reaping the corn, [that is], into instruments used for peaceful purposes, and that they are now unaccustomed to fighting, but when smitten, offer also the other cheek.
Irenaeus (A.D. 180), Ante-Nicene Church Fathers – Vol. 1, Page 512.

For when they know that we cannot endure even to see a man put to death, though justly; who of them can accuse us of murder or cannibalism? …But we, deeming that to see a man put to death is much the same as killing him, have abjured such spectacles. How, then, when we do not even look on, lest we should contract guilt and pollution, can we put people to death?
Athenagorus (A.D. 137), Ante-Nicene Church Fathers – Vol. 2, Page 147.

For He says, “Take no anxious thought for tomorrow,” meaning that the man who has devoted himself to Christ ought to be sufficient to himself, and servant to himself, and moreover lead a life which provides for each day by itself. For it is not in war, but in peace, that we are trained. War needs great preparation, and luxury craves profusion; but peace and love, simple and quiet sisters, require no arms nor excessive preparation. The Word is their sustenance.
Clement of Alexandria (A.D. 195), Ante-Nicene Church Fathers – Vol. 2, Page 235.

In their wars, therefore, the Etruscans use the trumpet, the Arcadians the pipe, the Sicilians the pectides, the Cretans the lyre, the Lacedaemonians the flute, the Thracians the horn, the Egyptians the drum, and the Arabians the cymbal. The one instrument of peace, the Word alone by which we honor God, is what we employ. We no longer employ the ancient psaltery, and trumpet, and timbrel, and flute, which those expert in war and despisers of the fear of God were wont to make use of also in the choruses at their festive assemblies; that by such strains they might raise their dejected minds.
Clement of Alexandria (A.D. 195), Ante-Nicene Church Fathers – Vol. 2, Pages 248-249.

For we do not train our women like Amazons to manliness in war; since we wish the men even to be peaceable. I hear that the Sarmatian women practice war no less than the men; and the women of the Sacae besides, who shoot backwards, feigning flight as well as the men. I am aware, too, that the women near Iberia practice manly work and toil, not refraining from their tasks even though near their delivery; but even in the very struggle of her pains, the woman, on being delivered, taking up the infant, carries it home.
Clement of Alexandria (A.D. 195), Ante-Nicene Church Fathers – Vol. 2, Page 420.

Above all, Christians are not allowed to correct with violence the delinquencies of sins. For it is not those that abstain from wickedness from compulsion, but those that abstain from choice, that God crowns. It is impossible for a man to be steadily good except by his own choice. For he that is made good by compulsion of another is not good; for he is not what he is by his own choice. For it is the freedom of each one that makes true goodness and reveals real wickedness. Whence through these dispositions God contrived to make His own disposition manifest.
Clement of Alexandria (A.D. 195), Ante-Nicene Church Fathers – Vol. 2, Page 581.

But now inquiry is made about this point, whether a believer may turn himself unto military service, and whether the military may be admitted unto the faith, even the rank and file, or each inferior grade, to whom there is no necessity for taking part in sacrifices or capital punishments. There is no agreement between the divine and the human sacrament, the standard of Christ and the standard of the devil, the camp of light and the camp of darkness. One soul cannot be due to two masters— God and Cæsar. And yet Moses carried a rod, and Aaron wore a buckle, and John (Baptist) is girt with leather and Joshua the Son of Nun leads a line of march; and the People warred: if it pleases you to sport with the subject. But how will a Christian man war, nay, how will he serve even in peace, without a sword, which the Lord has taken away? For albeit soldiers had come unto John, and had received the formula of their rule; albeit, likewise, a centurion had believed; still the Lord afterward, in disarming Peter, disarmed every soldier. No dress is lawful among us, if assigned to any unlawful action. -Tertullian On Idolatry Chapter 19

I think we must first inquire whether warfare is proper at all for Christians. What sense is there in discussing the merely accidental, when that on which it rests is to be condemned? Do we believe it lawful for a human oath to be superadded to one divine, for a man to come under promise to another master after Christ, and to abjure father, mother, and all nearest kinsfolk, whom even the law has commanded us to honor and love next to God Himself, to whom the gospel, too, holding them only of less account than Christ, has in like manner rendered honor? Shall it be held lawful to make an occupation of the sword, when the Lord proclaims that he who uses the sword shall perish by the sword? And shall the son of peace take part in the battle when it does not become him even to sue at law?
Tertullian (A.D. 198), Ante-Nicene Church Fathers – Vol. 3, Page 99.

Of course, if faith comes later, and finds any preoccupied with military service, their case is different, as in the instance of those whom John used to receive for baptism, and of those most faithful centurions, I mean the centurion whom Christ approves, and the centurion whom Peter instructs; yet, at the same time, when a man has become a believer, and faith has been sealed, there must be either an immediate abandonment of it, which has been the course with many; or all sorts of quibbling will have to be resorted to in order to avoid offending God, and that is not allowed even outside of military service; or, last of all, for God the fate must be endured which a citizen-faith has been no less ready to accept. Neither does military service hold out escape from punishment of sins, or exemption from martyrdom.
Tertullian (A.D. 198), Ante-Nicene Church Fathers – Vol. 3, Page 100.

Unless those physical wars bore the figure of spiritual wars, I do not think the books of Jewish history would ever have been handed down by the apostles to the disciples of Christ, who came to teach peace, so that they could be read in the churches. For what good was that description of wars to those to whom Jesus says, “My peace I give to you; my peace I leave to you,” and to whom it is commanded and said through the Apostle, “Not avenging your own selves,” and, “Rather, you receive injury,” and, “You suffer offense”? In short, knowing that now we do not have to wage physical wars, but that the struggles of the soul have to be exerted against spiritual adversaries, the Apostle, just as a military leader, gives an order to the soldiers of Christ, saying, “Put on the armor of God, so that you may be able to stand firm against the cunning devices of the Devil.” And in order for us to have examples of these spiritual wars from deeds of old, he wanted those narratives of exploits to be recited to us in church, so that, if we are spiritual—hearing that “the Law is spiritual”—“we may compare spiritual things with spiritual” in the things we hear. And we may consider, by means of those nations that fought visibly against physical Israel, how great are the swarms of opposing powers from among the spiritual races that are called “spiritual wickedness in the heavens,” and that stir up wars against the
Lord’s Church, which is the true Israel.
Origen (A.D. 248), Commentary on Joshua, Homily 15, paragraph 1

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;” [1 Timothy 2:1,2] 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.
Origen (A.D. 248), Ante-Nicene Church Fathers – Vol. 4, Pages 668-669.

They will inquire concerning the works and occupations of those are who are brought forward for instruction…. A military man in authority must not execute men. If he is ordered, he must not carry it out. Nor must he take military oath. If he refuses, he shall be rejected. If someone is a military governor, or the ruler of a city who wears the purple, he shall cease or he shall be rejected. The catechumen or faithful who wants to become a soldier is to be rejected, for he has despised God.
Hippolytus (A.D. 215), Apostolic Tradition, chapter 16

“To us it is not lawful either to see or to hear of homicide; and so much do we shrink from human blood, that we do not use the blood even of eatable animals in our food.”
Mark Minucius Felix, Octavius, chapter 30

“On a certain occasion, when a large body of captives were offered to the bishop Archelaus by the soldiers who held the camp in that place, their numbers being some seven thousand seven hundred, he was harassed with the keenest anxiety on account of the large sum of money which was demanded by the soldiers as the price of the prisoners’ deliverance. And as he could not conceal his solicitude, all aflame for the religion and the fear of God, he at length hastened to Marcellus, and explained to him the importance and difficulty of the case. And when that pattern of piety, Marcellus, heard his narration, without the least delay he went into his house, and provided the price demanded for the prisoners, according to the value set upon them by those who had led them captive; and unlocking the treasures of his goods, he at once distributed the gifts of piety among the soldiers, without any severe consideration of number or distinction, so that they seemed to be presents rather than purchase-moneys. And those soldiers were filled with wonder and admiration at the grandeur of the man’s piety and munificence, and were struck with amazement, and felt the force of this example of pity; so that very many of them were added to the faith of our Lord Jesus Christ, and threw off the belt of military service, while others withdrew to their camp, taking scarcely a fourth part of the ransom, and the rest made their departure without receiving even so much as would defray the expenses of the way.”
Archelaus, Acts of the Disputation with Manes, chapter 1

For when God forbids us to kill, He not only prohibits us from open violence, which is not even allowed by the public laws, but He warns us against the commission of those things which are esteemed lawful among men. Thus it will be neither lawful for a just man to engage in warfare, since his warfare is justice itself, nor to accuse any one of a capital charge, because it makes no difference whether you put a man to death by word, or rather by the sword, since it is the act of putting to death itself which is prohibited. Therefore, with regard to this precept of God, there ought to be no exception at all; but that it is always unlawful to put to death a man, whom God willed to be a sacred animal.
Lactantius (c. 304-313, W), Divine Institutes, book 6, chapter 20

Abstain from fleshly and worldly lusts. If any one gives you a stroke on your right cheek, turn to him the other also. Not that revenge is evil, but that patience is more honourable. For David says, If I have made returns to them that repaid me evil. If anyone compels you to go one mile, go with him two. And, He that will sue you at the law, and take away your coat, let him have your cloak also. Matthew 5:40; Luke 6:29 And from him that takes your goods, require them not again. Luke 6:30 Give to him that asks you, and from him that would borrow of you do not shut your hand. Matthew 5:42 For the righteous man is pitiful, and lends. For your Father would have you give to all, who Himself makes His sun to rise on the evil and on the good, and sends His rain on the just and on the unjust. Matthew 5:45 It is therefore reasonable to give to all out of your own labours; for says He, Honour the Lord out of your righteous labours, but so that the saints be preferred. You shall not kill; that is, you shall not destroy a man like yourself: for you dissolve what was well made. Not as if all killing were wicked, but only that of the innocent: but the killing which is just is reserved to the magistrates alone. – Apostolic constitutions, book 7, Section 1, Chapter 2

They were so alarmed that they… pretended an unseemly excuse, that they were not able to come now on account of the war which was begun by the Persians. But this was not the true cause of their delay, but the fears of their own consciences. For what have Bishops to do with war? – Athanasius, History of the Arian Controversy, chapter 11

Now Julian, once he had come into the east to drive out the Persians by war and begun to be carried away by the unconcealed craze for idolatry which earlier he had kept secret, showed himself more astute than the others as a persecutor in that he ruined almost more people by rewards, honors, flattery, and persuasion, than if he had proceeded by way of force, cruelty, and torture. Forbidding Christians access to the study of the pagan authors, he decreed that elementary schools should be open only to those who worshiped the gods and goddesses. He ordered that posts in the [armed or civil] service should be given only to those who sacrificed. He decreed that the government of provinces and the administration of justice should not be entrusted to Christians, since their own law forbade them to use the sword. – Rufinus, Church History, book 10, chapter 33

For Christians above all men are not permitted forcibly to correct the failings of those who sin. Secular judges indeed, when they have captured malefactors under the law, show their authority to be great, and prevent them even against their will from following their own devices: but in our case the wrong-doer must be made better, not by force, but by persuasion. Chrysostom, On the Priesthood, Book 2