Ron Sider (If Jesus Is Lord) doesn't think the New Testament episodes about soldiers tell us anything much about the Christian ethics of war.
He acknowledges that John doesn't tell the soldiers who come to him that they have to abandon their profession. BUt that doesn't matter, since "the precursor of Jesus does not have the same authority as Jesus" and "John the Baptist was not a Christian" (80). Jesus radically changes the ethics of war in a way that John didn't.
Sider also says arguments about soldiers are arguments from silence: "It is just as plausible to argue that Jesus and Peter also told the centurions to stop being soldiers as to argue that they conveyed the message that continuing as soldiers was quite ethical. The texts say absolutely nothing about either" (80).
That's technically true, but if soldiering were inherently wicked on par, say, with sorcery, we would expect to hear something. After all, as Sider notes, converted sorcerers burn their books of magic (79). It's odd we don't have a single scene of a converted soldier discarding his armor, sword, or spear. If the writers of the Gospels and Acts were opposed to war on principle, this silence is very odd. Would Sider leave it an open question whether or not converts stayed in the army? Why do the apostles?
Sider also says: Jesus welcomes prostitutes and tax collectors (81). That doesn't mean He endorses those professions. The fact that Jesus and Peter receive Roman soldiers doesn't imply they approve of the military life.
The analogy doesn't hold. We know some tax collectors repented dramatically (Zaccheus and his fourfold restitution) and Jesus tells sinful women to "go and sin no more." It's once again odd that neither Jesus nor any of the apostles ever requires a converted soldier to do anything similar.
Nigel Biggar has the better of the biblical argument (In Defence of War, 40-42). He sees the narratives of converted soldiers as the "thin edge of the wedge" in the argument against pacifism.
Like Sider, Richard Hays dismisses these episodes as negligible. Biggar notes in response to Hays: "What he means by this is that the New Testament is so predominantly in favour of non-violence that these soldier stories may be discounted." But, "the fact that they need to be discounted at all, of course, implies that, notwithstanding his attempt to tame them, Hays recognizes that they continue to carry non-pacifist significance."
The narratives of soldier conversions indicate that the New Testament is not nearly so "unambiguous" in opposition to war as Sider, Hays, and others claim. From that starting point, Biggar goes on to question the pacifist view that Jesus issued a blanket prohibition against military service and war. I agree with Biggar.
Sider's other argument is historical: "every single Christian author [before Constantine] who raises the question of killing says Christians should not do that. Every Christian author who discusses whether Christians should join the Roman army says they should not" (81).
Sider has compiled early church testimonies in The Early Church on Killing (ECK), which the subtitle describes as a "comprehensive sourcebook" on the subject.
It's an impressive collection of evidence. Sider argues that every Christian writer condemned all killing. But he, admirably, assembles the conclusive evidence that Christians did actually serve in the Roman army.
He cites the episode of the Marcus Aurelius's miraculous victory in 176, which some Christian writers (Tertullian and Apollinarius) attributed to the prayers of Christian soldiers. Sider rightly concludes that "it is highly likely . . . that there were Christian soldiers present at this important battle" and that "by the latter part of the second century there were some Christians serving in the Roman army" (ECK, 137). He's right to add that we don't know how many there were. This episode, he adds, doesn't prove that "large numbers of Christians considered killing, or service in the army, legitimate" (ECK, 138). But that reference to "large numbers" is an important qualification to his more absolute statements elsewhere. The evidence leaves room to conclude that some Christians weren't pacifists.
In a section on military martyrs, he notes that "the number of Christians in the Roman army increased significantly in the latter half of the third century" (ECK, 151). During the reign of Diocletian, "there were . . . increasing numbers of Christians in the army" (ECK, 151). This evidence is, importantly, prior to Constantine.
Sider concludes there was "unanimous teaching" from "all extant Christian writers" that killing is wrong in all circumstances. Yet there's also "clear evidence that more and more Christians were in the army" (ECK, 193). He's not sure how to explain this "disconnect." It wouldn't be the last time that "Christian laity have not lived what Christian leaders have taught" (ECK, 194).
Sider's interpretation of the historical evidence depends on his understanding of Jesus as a teacher of absolute non-violence, which I dispute. For instance, he quotes a passage from Clement that's often used to claim he thought military service legitimate. The passage (from Exhortation to the Greeks) says, "Has knowledge taken hold of you while engaged in military service? Listen to the commander, who orders what is right" (quoted ECK, 35).
Sider thinks that the "commander" is Jesus; Clement thus means that "soldiers who become Christians must obey Christ" (ECK, 34). But that only means "give up military service" if Sider is right about Jesus' teaching. Clement does not say, or even imply, that converted soldiers need to leave the army. He could well be saying that they obey Jesus as soldiers.
In general, there's another way to view the evidence of the early church. If we don't assume Jesus taught pacifism, and if we do give weight to the soldier converts in the New Testament, then it's plausible to see continuity from the first to the fourth century. Christian soldiers were there the whole time - warned against to restrain themselves, to renounce unjust force, abuse of power, idolatry - but still soldiers.