On Idolatry by Tertullian
Chapter XIX.--Concerning Military Service.
In that last section, decision may seem to have been given likewise concerning military service, which is between dignity and power. 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 Cæsar. And yet Moses carried a rod, and Aaron wore a buckle, and John (Baptist) is girt with leather and Joshua the son of Nun leads a line of march; and the People warred: if it pleases you to sport with the subject. 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, unbe**d every soldier. No dress is lawful among us, if assigned to any unlawful action.