In modern times it may not even seem like an issue if a Christian can or cannot serve in the military, clearly Christians today and for many centuries do, so what would the issue be? On the other hand, anyone who has honestly read the gospels must recognize that the issue of violence is at the very least problematic in Jesus’s teachings. One would simply have to point to the Sermon on the Mount (Matthew 5:38–47) or the Sermon on the Plain (Luke 6:27–31), or his teaching on the one taking the sword dying by the sword (Matthew 26:52), to recognize that Jesus tended to reject violence.
There is of course Augustine’s Just War theory, which was expanded by Thomas Aquinas. Much of this Just war theorizing is based on Romans 13. However, as John Howard Yodar has masterfully pointed out in his The Politics of Jesus, Paul assumes that the governing authorities are completely distinct from the audience of Romans 13; whereas what is found in Romans 12:9–21, right before Romans 13, actually lays out the ethical norms for a Christian. Romans 13 is not normative, it describes the reality, and discourages rebellions, whereas Romans 12 is normative. So Romans 13 cannot be used as a justification for a Christian doing violence in the military. Even so, as David Bentley Hart pointed out in commonweal, the word μάχαιρα is a term that could be used to refer to a civil guard, police officer or something like that as opposed to ξίφος, which refers to a sword used to execute people.
How about the early Church though, there is evidence that people who were soldiers became Christians very early on, although it’s not clear what happened after they became Christians, but evidence for Christians serving in the military comes at the end of the second century, and that evidence is very meager. However, the early Christian literature on the matter is almost unanimous, for example, Justin Martyr says:
“For from Jerusalem there went out into the world, men, twelve in number, and these illiterate, of no ability in speaking: but by the power of God they proclaimed to every race of men that they were sent by Christ to teach to all the word of God; and 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.”
Tertullian, echoing the same sentiment says:
“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 Caesar.
. . .
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.”
In another writing Tertullian says:
“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? And shall he apply the chain, and the prison, and the torture, and the punishment, who is not the avenger even of his own wrongs?
. . .
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.
. . .
Indeed, if, putting my strength to the question, I banish from us the military life.”
These sentiments are echoed in other early Christian writers such as Origen and Clement of Alexandria … they are, basically, unanimous: Christians do not serve in the military, they don’t go to war, and they don’t kill under any circumstances—no exceptions. Caesar does what Caesar does, but for the early Christians, that had no bearing on Christian ethics. The interpretation of Jesus disarming Peter is clear for Tertullian: Christians are not to take up the sword. Other writers such as Clement of Alexandria and Athenagoras also tie Christian pacifism directly to Jesus’s teachings. The aversion to military service was so strong for many Christians that we have accounts of many Christians preferring the death penalty to military service; such as Maximillian in 295 C.E.; or Marcellus 3 years later, who actually was a centurion but quit, on pains of death, his service due to his Christian convictions.
The record is clear; Christians for the first few centuries of their history were opposed to military service, war, and violence, no matter who sanctioned it.
This should not be surprising, there is no reason why we shouldn’t assume the early Christians took Jesus’s commands to “turn the other cheek” or “love your enemies” or “happy are the peacemakers” literally. For the first couple of centuries the Christians had no power over the state, nor did they seek it; they had no allegiance to the state, in fact, they shunned it. Given this, it is absolutely clear and logical why the early Christians were more or less pacifists. What changed? Well one thing is simply power, Augustine in Contra Faustum Manichaeum 22 writes:
“For there is no power but of God, Romans 13:1 who either orders or permits. Since, therefore, a righteous man, serving it may be under an ungodly king, may do the duty belonging to his position in the State in fighting by the order of his sovereign — for in some cases it is plainly the will of God that he should fight, and in others, where this is not so plain, it may be an unrighteous command on the part of the king, while the soldier is innocent, because his position makes obedience a duty, — how much more must the man be blameless who carries on war on the authority of God, of whom every one who serves Him knows that He can never require what is wrong?”
And a little later:
“we have seen Christian emperors, who have put all their confidence in Christ, gaining splendid victories over ungodly enemies, whose hope was in the rites of idolatry and devil-worship. There are public and undeniable proofs of the fact, that on one side the prognostications of devils were found to be fallacious, and on the other, the predictions of saints were a means of support; and we have now writings in which those facts are recorded.”
In the first quote, we see the argument from Romans 13, which completely ignores Romans 12. The second quote is I think ultimately the issue, at this point in time, there were Christian Emperors, they fought wars—so unless we are going to proclaim them sinful in their waging of wars and call for them to either dismantle the empire or give up their power, we have to accept that these wars were ordained by God. Another issue for Augustine is his extreme anti-Pelagianism and his spiritualizing of Christian ethics, for example earlier in Contra Faustum Manichaeum he writes:
“The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power, and such like; and it is generally to punish these things, when force is required to inflict the punishment, that, in obedience to God or some lawful authority, good men undertake wars, when they find themselves in such a position as regards the conduct of human affairs, that right conduct requires them to act, or to make others act in this way.”
According to Augustine, Jesus’s ethics are not about action, they are about internal disposition. It is clearly the case that Jesus did care about internal disposition, however it is not the case that he didn’t care about action or that he thought internal disposition could be separated from action. Augustine here, and in much of his ethics, is simply overturning what the standard interpretation of Christian ethics had been, and what the teachings of Jesus clearly were, to make Christianity safe for the Roman Empire. Nevertheless, pre-Augustine the standards were clear, Jesus expected his followers to shun violence, Paul opposed violence, and the early Fathers opposed violence; including violence done through the military.
 Justin Martyr, Apology, 39.
 Tertullian, On Idolatry, 19.
 Tertullian, De Corona, 11.
 Augustine, Contra Faustum Manichaeum 22.75
 Augustine, Contra Faustum Manichaeum 22.76
 Augustine, Contra Faustum Manichaeum 22.74
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