Excerpt taken from The Kingdom of God, Volume 3: Learning War No More by Tom A. Jones, used with permission
As we pursue Kingdom living and obedience to Jesus, our ultimate authority is found in his words and the words of Scripture. However, church history is still a useful tool. And so, we ask, what do we find about Christians and the military in the times closest to Jesus’ life—the second and third century centuries? Do we find that those Christians in the first two or three centuries really embraced the principle of peace or did they soon depart from this most impractical way of living? Was it found to be an impossible ethic?
Fortunately, we have the works of those we often call the church fathers. These leaders left us with writings and teachings on a variety of subjects including the one we are addressing in this book. The Roman Empire had a powerful world presence, and a robust military was a key to their success. The early church grew and spread in this environment. The invasion of the Kingdom of God clashed with the power of Rome on many points, and the works of the church fathers illustrate that Kingdom ideas about enemies and the military were in sharp contrast with those of the empire.
We can agree that these believers were not writing Scripture. We are not basing our lives and the practice of our churches on Justin, Clement, Tertullian or Origen. They believed and taught some things that I do not believe and will not teach today. So, why does it matter what they taught and did regarding soldiering and war?
Let us say clearly that it is not enough for us to be Christian pacifists because that is what the church of the first three centuries taught and practiced. We should only take that position if it fits with the message of Jesus and is a part of living out his Kingdom in the here and now. However, I find it significant that in a violent world where they were often the object of the violence, Christians of the first three hundred years held on to this non-violent message and only gave it up when their faith and the politics of this world, with its “wisdom,” were mixed together beginning with the emperor Constantine who brought church and empire together in an unholy alliance.
In some cases, it appears the early church made decisions for worldly reasons. In this case, where the church held to non-violence, it seems clear that they made this decision purely because they were seeking to be faithful to Jesus as Lord and to do God’s will on earth as it is in heaven. There was no earthly reason and no argument from philosophy that would have led them to this commitment. It appeared weak, foolish and irrational. It was ridiculed. But when the world’s wisdom and God’s wisdom clashed, they held on to the “foolishness” of God’s wisdom—at least in this case.
Quite often the early church writers put the emphasis on Jesus’ words of peace, non-resistance and enemy-love from the Sermon on the Mount. At other times they quote from Isaiah (or Micah) about swords being transformed into plowshares to show that the Kingdom life is oriented towards peace.
One of the earliest extra-biblical Christian works actually is from the late part of the first century and is known as the Didache, or by its other title, The Teachings of the Twelve Apostles. We don’t know its author (or authors) but scholars date it between 80 and 90 AD. The opening words of this echo the words of Jesus in Matthew’s gospel and we can see that the church was putting front and center Jesus’ challenging teaching about how to treat an enemy.
“This is the way of life: first, you shall love the God who made you, secondly, your neighbor as yourself: and all things whatsoever you would not should happen to you, do not thou to another. The teaching of these words is this: Bless those who curse you, and pray for your enemies, and fast on behalf of those who persecute you: for what thanks will be due to you, if you love only those who love you? Do not the Gentiles also do the same? But love those who hate you, and you shall not have an enemy.” 
Clement of Alexandria writing about 195 AD, said, “He bids us to ‘love our enemies, bless those who curse us, and pray for those who despitefully use us.’ He elaborates on Jesus’ words when he writes, ‘If anyone strikes you on the one cheek, turn to him the other also, and if anyone takes away your coat, do not hinder him from taking your cloak also.’ An enemy must be aided so that he may not continue as an enemy. For by help, good feeling is compacted and enmity dissolved.” 
Justin Martyr (100AD – 165AD) was born in Palestine, but died for his faith in Rome. He alludes to Isaiah 2 when he says, “We ourselves were well conversant with war, murder and everything evil, but all of us throughout the whole wide earth have traded in our weapons of war. We have exchanged our swords for plowshares, our spears for farm tools. Now we cultivate the fear of God, justice, kindness, faith, and the expectation of the future given us through the Crucified One…. The more we are persecuted and martyred, the more do others in ever increasing numbers become believers.” Later he added: “We who formerly treasured money and possessions more than anything else, now hand over everything we have to a treasury for all and share it with everyone who needs it. We who formerly hated and murdered one another now live together and share the same table. We pray for our enemies and try to win those who hate us.”
One of the more interesting documents that we have from the late 100s or early 200s is something known as the Apostolic . Most scholars believe it is written by Hippolytus of Rome (170 – 236) and it is basically a manual for church life including instructions about worship. It contains this passage that is relevant for us:
“The professions and trades of those who are going to be accepted into the community must be examined. The nature and type of each must be established.” Then is listed different professions that must be given up by Christians, including: keeper of a brothel, sculptor of idols, charioteer, athlete, gladiator, and each time it states “give it up or be rejected.” Then we have this: “A military constable must be forbidden to kill, neither may he swear; if he is not willing to follow these instructions, he must be rejected. A proconsul or magistrate who wears the purple and governs by the sword shall give it up or be rejected.”
And then a bit later he writes:
“Anyone taking part in baptismal instruction or already baptized who wants to become a soldier shall be sent away, for he has despised God.” Another version of this document adds this: “A soldier in the sovereign’s army should not kill or if he is ordered to kill, he should refuse. If he stops, so be it; otherwise, he should be excluded.” Other practices that are then listed as unacceptable: prostitute, sodomite, magician, and soothsayer. All these must be given up.” 
Preston Sprinkle argues that the Apostolic Traditions is important because it shows us what was going on down at the local church level and not just what was being taught by the church theologians. 
Cyprian (200 –258), who was the bishop of Carthage in North Africa compared killing in war with murder with this pointed comment: “We are scattered over the whole earth with the bloody horror of camps. The whole world is wet with mutual blood. and murder—which is admitted to be a crime in the case of an individual—is called a virtue when it is committed wholesale. Impunity is claimed for wicked deeds, not because they are guiltless—but because the cruelty is perpetrated on a grand scale!” For Cyprian, as for most early church writers and leaders, the problem was killing, and he found it odd that killing an individual in daily life is universally rejected while the wholesale killing that goes on in war is exalted.
In another place he addressed the enemies of the faith: “None of us offers resistance when he is seized, or avenges himself for your unjust violence, although our people are numerous and plentiful…it is not lawful for us to hate, and so we please God more when we render no requital for injury…We repay your hatred with kindness.” In this, he found the uniqueness of Jesus’ ethic.
1 Didache, I, 2-3
2 David Bercot, editor, A Dictionary of Early Christian Beliefs (Peabody, Mass.: Hendrickson Publishing, 1998), 677. Unless otherwise noted all quotes from the early Christian writers will be from this volume which quotes from the Anti-Nicene Fathers which is also published by Hendrickson Publishing.
3 Bercot, A Dictionary of Early Christian Beliefs, 676.
4 Eberhard Arnold, editor, The Early Christians in Their Own Words (Walden, NY: Plough Publishing, 1997), 90.
5 Sprinkle, Preston. Fight: A Christian Case for Non-Violence (Elgin, Illinois: David C. Cook, 2017) Kindle Location 3143-3165.
6 Bercot, A Dictionary of Early Christian Beliefs, 680.
7 Testamonia, iii, 106.