Excerpt taken from The Kingdom of God, Volume 3: Learning War No More by Tom A. Jones, used with permission
Tertullian (160 –220) was raised by pagan parents, and he did not become a Christian until sometime in his late thirties or early forties. Nevertheless, he became one of the most prolific writers in the early church. He is sometimes called the “Father of Western Theology” and the “The Father of Latin Christianity.” He was the first of these leaders to write most of his works in Latin. He eventually left the mainstream church and became part of the Montanist movement because he felt the majority was turning in a worldly direction.
Particularly after he joined the Montanists, he wrote much about the need for Christians to avoid war. He wrote an entire treatise titled The Crown, in which he spelled out the Christian case against military involvement. Here is a sampling of the points that he made in that work and in other writings:
“If then, we are commanded to love our enemies, whom have we to hate? If injured, we are forbidden to retaliate, lest we become just as bad ourselves who can suffer injury at our hands.” 
“Only without the sword can the Christian wage war: the Lord has abolished the sword. Christ, in disarming Peter, disarmed every soldier.” 
“But now inquiry is being made concerning these issues. First, can any believer enlist in the military? Second, can any soldier, even those of the rank and file or lesser grades who neither engage in pagan sacrifices nor capital punishment, be admitted into the church? No on both counts—for there is no agreement between the divine sacrament 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 serve two masters—God and Caesar…But how will a Christian engage in war (indeed, how will a Christian even engage in military service during peacetime) without the sword, which the Lord has taken away?” 
“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?” 
“Will those who are forbidden to engage in a lawsuit espouse the deeds of war? Will a Christian who is told to turn the other cheek when struck unjustly, guard prisoners in chains, and administer torture and capital punishment?” 
Tertullian’s arguments revolved around three points: (1) Jesus’ command for us to love enemies, (2) Jesus’ call for Peter to put down the sword while condemning those who lived by the sword, and (3) Jesus’ overall ethic involving non-resistance and non-retaliation.
Sometimes it is said that Tertullian and other writers were mainly concerned about the idolatry that was a consistent part of Roman military experience—something that would not be found in the military today. There is no doubt that this was a concern, and Tertullian addresses that in The Crown and in On Idolatry, but from the quotes we have here, we can see that his objections to the military were not limited to that concern.
Some who don’t believe Tertullian to be useful in this discussion often point out that he also forbids a Christian to be a schoolmaster, a teacher of literature, a seller of frankincense, and that he condemns all forms of painting, modelling and sculpture. The argument is if he was wrong about these, then he was also wrong about the military. However, one must consider how biblically based was his critique of military service while his condemnation of other things was more based on his general concern for how far the church was drifting in worldly directions.
Origen (185–254 ) was even more prolific than Tertullian, writing an astonishing 2,000 plus treatises on nearly every biblical subject imaginable. His works contain a great many comments related to our topic. Here are some of those:
“Yet Christ nowhere teaches that it is right for his disciples to offer violence to anyone, no matter how wicked. For he did not consider it to be in accord with his laws. To allow for the killing of any individual whomever for his laws are derived from a Divine source ... For his laws do not allow them on any occasion to resist their persecutors even when it is their fate to be slain as sheep.” 
“To those who ask us whence we have come or whom we have for a leader, we say that we have come in accordance with the counsels of Jesus to cut down our warlike and arrogant swords of argument into ploughshares, and we convert into sickles the spears we formerly used in fighting. For we no longer take ‘sword against a nation,’ nor do we learn ‘any more to make war,’ having become sons of peace for the sake of Jesus, who is our leader, instead of following the ancestral customs in which we were strangers to the covenants.” 
“To this our answer is, we do give help to Kings when needed. But this is so to speak, a Divine help, ‘putting on the whole armor of God.’” Origin then mentions Paul's command for us to pray for those in authority and ends with this statement: “This is a greater help than what is done by soldiers who go forth to kill as many of the enemy as they can.” 
“How was it possible for the gospel doctrine of peace to prevail throughout the world? For it does not permit men to take vengeance even on their enemies.” 
The man who has been described as “the greatest genius the early church ever produced” consistently opposed Christian participation in military activities. He was tortured for his faith during the Decian persecution in 250 and died three to four years later from his injuries.
Lactantius (c.250–c.325) is our main example of a church father whose early writing occurred before Constantine and his later work after the edict of Constantine in the new era when the Christian Church was viewed favorably by the empire. In his Divine Institutes, written in the earlier period, he makes as strong a statement as we can find, writing, “It is not right for a just man to serve in the army . . . Nor is it right for a just man to charge someone with a capital crime. It does not matter whether you kill a man with the sword or with a word, since it is killing itself that is prohibited. So, there must be no exception to this command of God. Killing a human being whom God willed to be inviolable [some translations: “a sacred animal”] is always wrong.” 
After Constantine experiences his “conversion” and gives the edict legalizing Christianity, he asked Lactantius to become his spiritual advisor, tutor his son, and help shape the church’s relationship to the empire. At some point we find Lactantius changing his posture. In his Epitome, he writes: “Just as courage is good, if you are fighting for your country but evil if you are rebelling against it, so too with the emotions. If you use them for good ends, they will be virtues; if for evil ends, they will be called vices.”  In other places we see that he no longer opposed all violence. Pacifists see this as an example of letting politics take priority over conviction. They see how gaining power has a corrupting effect and here is an example of what happened to the church in general in the era of Constantine. Others argue he was just faithfully adapting to new circumstances.
So, what is our conclusion? First, there is a united message from the church fathers of the first three centuries that the disciple of Jesus does not kill his enemy, but loves him. Let’s hear from Roland Bainton, the renowned church historian. In his Christian Attitudes toward War and Peace, he has a chapter titled “The Pacifism of the Early Church,” where he makes the following assertion: “The three Christian positions with regard to war . . . matured in chronological sequence, moving from pacifism to the just war to the Crusade. The age of persecution down to the time of Constantine was the age of pacifism to the degree that during this period no Christian author to our knowledge approved of Christian participation in battle.” 
1 Bercot, A Dictionary of Early Christian Beliefs, 677.
2 Bercot, 677.
3 Tertullian, De idololatria (Anti-Nicene Fathers 3:73).
4 Bercot, A Dictionary of Early Christian Beliefs, 678.
5 Bercot, 678.
6 Bercot, 678.
7 Bercot, 678.
8 Bercot, 679.
9 Bercot, 678.
10 Bercot, 681.
11 Epitome of the Divine Institutes, Chapter 61. http://www.newadvent.org/fathers/0702.htm.
12 Roland H. Bainton, Christian Attitudes toward War and Peace (New York, Abington, 1960), 66.