Here's an appeal I've run across from more than one pacifist author:
That claim suffers from multiple problems:
i) Unless you regard the church fathers are authority figures, that's an illicit argument from authority. And it's selective even on its own grounds: on the one hand, Origen and Tertullian aren't technically church fathers. On the other hand, it only selects for early (pre-Constantinian) church fathers.
ii) It also suffers from sample selection bias. It isn't polling early Christian opinion in general, but only Christian writers. There is, however, evidence, that early Christians did serve in the Roman army. For instance, Despina Iosif, in her monograph on Early Christian Attitudes to War, Violence, and Military Service (Gorgias Press 2013), cites epitaphs on the tombstones of Christian soldiers to show that military service was considered honorable among (at least some) early Christians.
So the appeal to early Christian writers preemptively discounts the views of Christians who were not writers, Christians who did, in fact, serve in the Roman army. But what makes the opinion of early Christian writers, rather than early Christians generally, the only evidence worthy of consideration?
Indeed, appeal to early Christian writers is self-defeating, for the reason they wrote about the subject in the first place was to discourage Christians from joining the Roman army. If, however, there was Christian consensus on the immorality of military service, their philippics would be superfluous.
iii) In addition, pacifists admit that early Christians might refuse military service, not because they thought killing was intrinsically wrong, but because military service was complicit with pagan rituals (e.g. the imperial cult). So that's another case of sample selection bias. You have to separate out those who oppose military service for pacifistic reasons from those who oppose it for other reasons. A person might support a defensive war, but not a war of conquest.
iv) Furthermore, it's fallacious to infer that refusal to volunteer for military service is equivalent to pacifism. Indeed, that's a category mistake.
a) That fails to distinguish between national defense and self-defense, or defensive wars and offensive wars.
When you join the military, you assume a risk. To some degree, you are putting yourself at greater risk than if you avoid military service.
It hardly follows that if someone puts you at risk (e.g. a mugger), you won't fight back. Indeed, the same reason some people avoid military service is why they will defend themselves if threatened: self-preservation. Just as they view a mugger as a threat to life and limb, they view military service as a threat to life and limb.
b) To approach this from another angle, in wartime, the state must often resort to military conscription. There aren't enough volunteers. And even then you have deserters and draft dodgers.
Again, though, that doesn't mean these people are pacifists. For instance, during the Civil War you had a significant percentage of Confederate deserters. But that's not because they were pacifists. If you invaded their farm, they'd shoot you dead. If you tried to harm their kin, they'd kill you. It's a question of where people draw the line.
c) During the Vietnam War, you had draftees who vowed never to kill anyone. It wasn't their cause. The containment policy was an abstraction. But when they got into theater, they did shoot the Viet Cong.
As long as they were at a safe distance, on American soil, the Viet Cong were not their enemies. The Viet Cong did not pose a direct mortal threat to their individual survival.
But when they were thrust into the field of battle, then the Viet Cong became their enemies. Suddenly, they had a personal stake in the outcome. If it's kill-or-be-killed, they will shoot you.