The Council of Chalcedon (451 AD)

We have decreed that those who have once been enrolled among the clergy, or have been made monks, shall accept neither a military charge nor any secular dignity; and if they shall presume to do so and not repent in such wise as to turn again to that which they had first chosen for the love of God, they shall be anathematized.

Ancient Epitome of Canon VII.

If any cleric or monk arrogantly affects the military or any other dignity, let him be cursed.

Something similar was ordered by the lxxxiii. (lxxxii.) Apostolic Canon, only that it threatens the cleric who takes military service merely with deposition from his clerical office, while our canon subjects him to excommunication.…The Greek commentators, Balsamon and Zonaras, think that our canon selects a more severe punishment, that of excommunication, because it has in view those clerics who have not merely taken military service, etc., but at the same time have laid aside their clerical dress and put on secular clothing.

By στρατείαν [which I have translated (or, as Canon Bright thinks, mistranslated) “military charge”], “militiam,” is here meant, not military employment as such, but the public service in general. This use of the term is a relic and token of the military basis of the Roman monarchy. The court of the Imperator was called his camp, στρατόπεδον, as in Constantine’s letter’s to John Archaph and the Council of Tyre, and in the VIIth canon of Sardica, so Athanasius speaks of the “camp” of Constans, and of that of Constantius at Milan (Hist. Ari., xxxvij.); so Hosius uses the same phrase in his letter to Constantius; so the Semi-Arian bishops, when addressing Jovian (Soz., vi. 4); so Chrysostom in the reign of Theodosius I. (Hom. ad Pop. Antioch, vi. 2). Similarly, there were officers of the palace called Castrensians, as being “milites alius generis—de imperatoria familia”. So στρατεύσθαι is used for holding a place at court, as in Soc., iv. 9; Soz., vi. 9, on Marcian’s case, and a very clear passage in Soc., v. 25, where the verb is applied to an imperial secretary. It occurs in combination with στρατεία in a petition of an Alexandrian deacon named Theodore, which was read in the third session of Chalcedon: he says, “᾽Εστρατευσάμεν for about twenty-two years in the Schola of the magistrians” (under the Magister officionum, or chief magistrate of the palace), “but I disregarded στρατείας τοσούτον χρόναυ in order to enter the ministry” (Mansi, vi. 1008). See also Theodoret, Relig. Hist., xij., on the emperor’s letter-carriers. In the same sense Honorius, by a law of 408, forbids non-Catholics “intra palatium militare” (Cod Theod., xvi., 5, 42); and the Vandal king Hunneric speaks of “domus nostræ militiæ” (Victor Vitens, iv. 2).

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa xx., Quæst. iii., Can. iij.