Objection 1. It would seem that no religious order can be directed to soldiering. For all religious orders belong to the state of perfection. Now our Lord said with reference to the perfection of Christian life (Matthew 5:39): "I say to you not to resist evil; but if one strike thee on the right cheek, turn to him also the other," which is inconsistent with the duties of a soldier. Therefore no religious order can be established for soldiering.
Objection 2. Further, the bodily encounter of the battlefield is more grievous than the encounter in words that takes place between counsel at law. Yet religious are forbidden to plead at law, as appears from the Decretal De Postulando quoted above (2, Objection 2). Therefore it is much less seemly for a religious order to be established for soldiering.
Objection 3. Further, the religious state is a state of penance, as we have said above (Question 187, Article 6). Now according to the code of laws soldiering is forbidden to penitents. for it is said in the Decretal De Poenit., Dist. v, cap. 3: "It is altogether opposed to the rules of the Church, to return to worldly soldiering after doing penance." Therefore it is unfitting for any religious order to be established for soldiering.
Objection 4. Further, no religious order may be established for an unjust object. But as Isidore says (Etym. xviii, 1), "A just war is one that is waged by order of the emperor." Since then religious are private individuals, it would seem unlawful for them to wage war; and consequently no religious order may be established for this purpose.
On the contrary, Augustine says (Ep. clxxxix; ad Bonifac.), "Beware of thinking that none of those can please God who handle war-like weapons. Of such was holy David to whom the Lord gave great testimony." Now religious orders are established in order that men may please God. Therefore nothing hinders the establishing of a religious order for the purpose of soldiering.
I answer that, As stated above (Article 2), a religious order may be established not only for the works of the contemplative life, but also for the works of the active life, in so far as they are concerned in helping our neighbor and in the service of God, but not in so far as they are directed to a worldly object. Now the occupation of soldiering may be directed to the assistance of our neighbor, not only as regards private individuals, but also as regards the defense of the whole commonwealth. Hence it is said of Judas Machabeus (1 Maccabees 3:2-3) that "he [Vulgate: 'they'] fought with cheerfulness the battle of Israel, and he got his people great honor." It can also be directed to the upkeep of divine worship, wherefore (1 Maccabees 3:21) Judas is stated to have said: "We will fight for our lives and our laws," and further on (1 Maccabees 13:3) Simon said: "You know what great battles I and my brethren, and the house of my father, have fought for the laws and the sanctuary."
Hence a religious order may be fittingly established for soldiering, not indeed for any worldly purpose, but for the defense of divine worship and public safety, or also of the poor and oppressed, according to Psalm 81:4: "Rescue the poor, and deliver the needy out of the hand of the sinner."
Reply to Objection 1. Not to resist evil may be understood in two ways. First, in the sense of forgiving the wrong done to oneself, and thus it may pertain to perfection, when it is expedient to act thus for the spiritual welfare of others. Secondly, in the sense of tolerating patiently the wrongs done to others: and this pertains to imperfection, or even to vice, if one be able to resist the wrongdoer in a becoming manner. Hence Ambrose says (De Offic. i, 27): "The courage whereby a man in battle defends his country against barbarians, or protects the weak at home, or his friends against robbers is full of justice": even so our Lord says in the passage quoted [Luke 6:30 "Of him that taketh away thy goods, ask them not again"; Cf. Matthew 5:40, "... thy goods, ask them not again." If, however, a man were not to demand the return of that which belongs to another, he would sin if it were his business to do so: for it is praiseworthy to give away one's own, but not another's property. And much less should the things of God be neglected, for as Chrysostom [Hom. v in Matth. in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says, "it is most wicked to overlook the wrongs done to God."
Reply to Objection 2. It is inconsistent with any religious order to act as counsel at law for a worldly object, but it is not inconsistent to do so at the orders of one's superior and in favor of one's monastery, as stated in the same Decretal, or for the defense of the poor and widows. Wherefore it is said in the Decretals (Dist. lxxxviii, cap. 1): "The holy synod has decreed that henceforth no cleric is to buy property or occupy himself with secular business, save with a view to the care of the fatherless... and widows." Likewise to be a soldier for the sake of some worldly object is contrary to all religious life, but this does not apply to those who are soldiers for the sake of God's service.
Reply to Objection 3. Worldly soldiering is forbidden to penitents, but the soldiering which is directed to the service of God is imposed as a penance on some people, as in the case of those upon whom it is enjoined to take arms in defense of the Holy Land.
Reply to Objection 4. The establishment of a religious order for the purpose of soldiering does not imply that the religious can wage war on their own authority; but they can do so only on the authority of the sovereign or of the Church.
Source: Summa Theologica (New Advent)