Objection 1. It would seem that religion does not imply a state of perfection. For that which is necessary for salvation does not seemingly pertain to perfection. But religion is necessary for salvation, whether because "thereby we are bound [religamur] to the one almighty God," as Augustine says (De Vera Relig. 55), or because it takes its name from "our returning [religimus] to God Whom we had lost by neglecting Him" [Cf. 81, 1], according to Augustine (De Civ. Dei x, 3). Therefore it would seem that religion does not denote the state of perfection.
Objection 2. Further, religion according to Tully (De Invent. Rhet. ii, 53) is that "which offers worship and ceremony to the Divine nature." Now the offering of worship and ceremony to God would seem to pertain to the ministry of holy orders rather than to the diversity of states, as stated above (40, 2; 183, 3). Therefore it would seem that religion does not denote the state of perfection.
Objection 3. Further, the state of perfection is distinct from the state of beginners and that of the proficient. But in religion also some are beginners, and some are proficient. Therefore religion does not denote the state of perfection.
Objection 4. Further, religion would seem a place of repentance; for it is said in the Decrees (VII, qu. i, can. Hoc nequaquam): "The holy synod orders that any man who has been degraded from the episcopal dignity to the monastic life and a place of repentance, should by no means rise again to the episcopate." Now a place of repentance is opposed to the state of perfection; hence Dionysius (Eccl. Hier. vi) places penitents in the lowest place, namely among those who are to be cleansed. Therefore it would seem that religion is not the state of perfection.
On the contrary, In the Conferences of the Fathers (Collat. i, 7) abbot Moses speaking of religious says: "We must recognize that we have to undertake the hunger of fasting, watchings, bodily toil, privation, reading, and other acts of virtue, in order by these degrees to mount to the perfection of charity." Now things pertaining to human acts are specified and denominated from the intention of the end. Therefore religious belong to the state of perfection.
Moreover Dionysius says (Eccl. Hier. vi) that those who are called servants of God, by reason of their rendering pure service and subjection to God, are united to the perfection beloved of Him.
I answer that, As stated above (Question 141, Article 2) that which is applicable to many things in common is ascribed antonomastically to that to which it is applicable by way of excellence. Thus the name of "fortitude" is claimed by the virtue which preserves the firmness of the mind in regard to most difficult things, and the name of "temperance," by that virtue which tempers the greatest pleasures. Now religion as stated above (81, 2; 3, ad 2) is a virtue whereby a man offers something to the service and worship of God. Wherefore those are called religious antonomastically, who give themselves up entirely to the divine service, as offering a holocaust to God. Hence Gregory says (Hom. xx in Ezech.): "Some there are who keep nothing for themselves, but sacrifice to almighty God their tongue, their senses, their life, and the property they possess." Now the perfection of man consists in adhering wholly to God, as stated above (Question 184, Article 2), and in this sense religion denotes the state of perfection.
Reply to Objection 1. To offer something to the worship of God is necessary for salvation, but to offer oneself wholly, and one's possessions to the worship of God belongs to perfection.
Reply to Objection 2. As stated above (81, 1, ad 1; 4, ad 1, 2; 85, 3) when we were treating of the virtue of religion, religion has reference not only to the offering of sacrifices and other like things that are proper to religion, but also to the acts of all the virtues which in so far as these are referred to God's service and honor become acts of religion. Accordingly if a man devotes his whole life to the divine service, his whole life belongs to religion, and thus by reason of the religious life that they lead, those who are in the state of perfection are called religious.
Reply to Objection 3. As stated above (184, 4,6) religion denotes the state of perfection by reason of the end intended. Hence it does not follow that whoever is in the state of perfection is already perfect, but that he tends to perfection. Hence Origen commenting on Matthew 19:21, "If thou wilt be perfect," etc., says (Tract. viii in Matth.) that "he who has exchanged riches for poverty in order to become perfect does not become perfect at the very moment of giving his goods to the poor; but from that day the contemplation of God will begin to lead him to all the virtues." Thus all are not perfect in religion, but some are beginners, some proficient.
Reply to Objection 4. The religious state was instituted chiefly that we might obtain perfection by means of certain exercises, whereby the obstacles to perfect charity are removed. By the removal of the obstacles of perfect charity, much more are the occasions of sin cut off, for sin destroys charity altogether. Wherefore since it belongs to penance to cut out the causes of sin, it follows that the religious state is a most fitting place for penance. Hence (XXXIII, qu. ii, cap. Admonere) a man who had killed his wife is counseled to enter a monastery which is described as "better and lighter," rather than to do public penance while remaining in the world.
Source: Summa Theologica (New Advent)