I want to present to you some very interesting writings by some of the church fathers regarding participation in the military. In doing so I am not claiming that the early church was monolithic in its stand against the use of violence, for if it were, the writers I'm about to cite might not have had to make their arguments at all. There have been at least some historical accounts of Christian soldiers during the first 2-3 centuries of the church--many (but not all) martyrs. Nevertheless I think the Christian-soldier apologists fail to appropriately reconcile the fact that, while some pre-Constantine church fathers are explicit in their opposition to war, and others may seem to acknowledge Christian soldiers as a reality, I have yet to see a single pre-fourth-century Christian writer who defended or advocated military service by the believer. An argument that military service may have been tolerated is not the same thing at all as claiming that it was encouraged.
A point of clarification before I proceed: I appeal to these writers, not because I suggest that they have Biblical or "inspired" authority, but rather as evidence of teaching within the early church, close to the time of Jesus. There is plenty within the writings of the very authors I quote, that I would consider quite weird (for example, the descriptions of pennance and preparation for baptism). Their testimony, however, is still compelling.
Tertullian (around 160-220 AD) was an early and prolific apologist for Christianity, hailing from Carthage. He had some interesting things to say about the use of the sword, which I offer below. These quotes are taken from the compilation "Latin Christianity: It's Founder - Tertullian" by Philip Schaff. The complete online text is available at the Christian Classics Ethereal Library. In each passage I have put the quote in green with added emphasis to key phrases in red.
(In answer to the accusation that Christians are the enemy of the state) If we are enjoined, then, to love our enemies, as I have remarked above, whom have we to hate? If injured, we are forbidden to retaliate, lest we become as bad ourselves: who can suffer injury at our hands?
(and later in the same passage)
For what wars should we not be fit, not eager, even with unequal forces, we who so willingly yield ourselves to the sword, if in our religion it were not counted better to be slain than to slay? Apologia, ch. 37
(Speaking of Jesus) He to whom, had He willed it, legions of angels would at one word have presented themselves from the heavens, approved not the avenging sword of even one disciple. The patience of the Lord was wounded in (the wound of) Malchus. And so, too, He cursed for the time to come the works of the sword; and, by the restoration of health, made satisfaction to him whom Himself had not hurt, through Patience, the mother of Mercy." De Patientia (On Patience), Ch. 3
And probably the most comprehensive:
To begin with the real ground of the military crown, 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 honour and love next to God Himself, to whom the gospel, too, holding them only of less account than Christ, has in like manner rendered honour? 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? Shall he, forsooth, either keep watch-service for others more than for Christ, or shall he do it on the Lord’s day, when he does not even do it for Christ Himself? And shall he keep guard before the temples which he has renounced? And shall he take a meal where the apostle has forbidden him? And shall he diligently protect by night those whom in the day-time he has put to flight by his exorcisms, leaning and resting on the spear the while with which Christ’s side was pierced? Shall he carry a flag, too, hostile to Christ? And shall he ask a watchword from the emperor who has already received one from God? Shall he be disturbed in death by the trumpet of the trumpeter, who expects to be aroused by the angel’s trump? And shall the Christian be burned according to camp rule, when he was not permitted to burn incense to an idol, when to him Christ remitted the punishment of fire? Then how many other offences there are involved in the performances of camp offices, which we must hold to involve a transgression of God’s law, you may see by a slight survey. The very carrying of the name over from the camp of light to the camp of darkness is a violation of it. 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. Nowhere does the Christian change his character. De Corona Militis (on the Military Crown), ch. 13
There is no question, in the broader context of De Idolatria and De Corona Militis, that one of Tertullian's major objections to military service was the frank idolatry that came with the territory. This is particularly clear in the last passage I quoted above. To argue, however, that idolatry was the only reason Tertullian had an issue with military service is to go against the plain language of his argument. He is abundantly clear that the use of the sword is contrary to the character expected of the Christian, and that trying to justify otherwise-sinful behavior because of the military command is "quibbling."
It is important to acknowledge here that Tertullian is not making any argument as to whether or not the state should have armies, or whether it should use them. He is merely arguing (but quite vehemently) that the follower of Jesus has no business taking part in this activity of the state. I suggest his argument still holds.