…Tertullian, for instance, wrote an entire treatise forbidding military service among Christians (The Crown) and such sentiment is found throughout his other writings (On Idolatry). Origen too condemned military service whenever he addressed the subject. And Lactantius agreed: “A just man may not be a soldier” (Divine Institutes, 6.20).
Now again, there were Christians in the military before Constantine. But as far as the opinion of early Christian writers goes, historian Alan Kreider was correct that “no Christian theologian before Constantine justified Christian participation in warfare” (“Military Service,” 431).
But this actually doesn’t tell us too much. The main question is not whether the theologians permit military service. This much is clear. They condemn it. The question, though, is why? On what grounds are Christians forbidden to join the military?…