But now the question is whether a believer can become a soldier and whether a soldier can be admitted into the faith, even if he is a member only of the rank and file who are not required to take part in sacrifices or capital punishments.

There can be no compatibility between the divine and the human sacrament (= military oath), the standard of Christ and the standard of the devil, the camp of light and the camp of darkness. One soul cannot serve two masters-God and Caesar. Moses, to be sure, carried a rod; Aaron wore a military belt, and John (the Baptist) is girt with leather (i.e., like a soldier); and, if you really want to play around with the subject, Joshua the son of Nun led an army and the people waged war. But how will a Christian man go to war? Indeed how will he serve even in peacetime without a sword which the Lord has taken away? For even if soldiers came to John and received advice on how to act, and even if a centurion became a believer, the Lord, in subsequently disarming Peter, disarmed every soldier, No uniform is lawful among us if it is designated for an unlawful action. (Treatise on Idolatry 19; Ante-Nicene Fathers 3:73)

 
Quoted in Helgeland, John, et al. Christians and the Military: The Early Experience. (Minneapolis: Augsburg, 1985).