The question is now whether a member of the faithful can become a soldier and whether a soldier can be admitted to the Faith even if he is a member of the rank and file who are not required to offer sacrifice or decide capital cases. There can be no compatibility between an oath made to God and one made to man, between the standard of Christ and that of the devil, between the camp of light and the camp of darkness. The soul cannot be beholden to two masters, God and Caesar. Moses, to be sure, carried a rod; Aaron wore a military belt and had a breast plate. If one wants to play around with the topic, Jesus (Joshua), son of Nun led an army and the Jewish nation went to war. But how will a Christian do so? Indeed how will he serve in the army even during peacetime without the sword that Jesus Christ has taken away? Even if soldiers came to John and got advice on how they ought to act, even if the centurion became a believer, the Lord, by taking away Peter’s sword, disarmed every soldier thereafter. We are not allowed to wear any uniform that symbolises a sinful act.
– Tertullian, On Idolatry