Objection 1. It would seem that religion does not direct man to God alone. It is written (James 1:27): "Religion clean and undefiled before God and the Father is this, to visit the fatherless and widows in their tribulation, and to keep oneself unspotted from this world." Now "to visit the fatherless and widows" indicates an order between oneself and one's neighbor, and "to keep oneself unspotted from this world" belongs to the order of a man within himself. Therefore religion does not imply order to God alone.
Objection 2. Further, Augustine says (De Civ. Dei x, 1) that "since in speaking Latin not only unlettered but even most cultured persons ere wont to speak of religion as being exhibited, to our human kindred and relations as also to those who are linked with us by any kind of tie, that term does not escape ambiguity when it is a question of Divine worship, so that we be able to say without hesitation that religion is nothing else but the worship of God." Therefore religion signifies a relation not only to God but also to our kindred.
Objection 3. Further, seemingly "latria" pertains to religion. Now "latria signifies servitude," as Augustine states (De Civ. Dei x, 1). And we are bound to serve not only God, but also our neighbor, according to Galatians 5:13, "By charity of the spirit serve one another." Therefore religion includes a relation to one's neighbor also.
Objection 4. Further, worship belongs to religion. Now man is said to worship not only God, but also his neighbor, according to the saying of Cato [Dionysius Cato, Breves Sententiae], "Worship thy parents." Therefore religion directs us also to our neighbor, and not only to God.
Objection 5. Further, all those who are in the state of grace are subject to God. Yet not all who are in a state of grace are called religious, but only those who bind themselves by certain vows and observances, and to obedience to certain men. Therefore religion seemingly does not denote a relation of subjection of man to God.
On the contrary, Tully says (Rhet. ii, 53) that "religion consists in offering service and ceremonial rites to a superior nature that men call divine."
I answer that, as Isidore says (Etym. x), "according to Cicero, a man is said to be religious from 'religio,' because he often ponders over, and, as it were, reads again [relegit], the things which pertain to the worship of God," so that religion would seem to take its name from reading over those things which belong to Divine worship because we ought frequently to ponder over such things in our hearts, according to Proverbs 3:6, "In all thy ways think on Him." According to Augustine (De Civ. Dei x, 3) it may also take its name from the fact that "we ought to seek God again, whom we had lost by our neglect" [St. Augustine plays on the words 'reeligere,' i.e. to choose over again, and 'negligere,' to neglect or despise.]. Or again, religion may be derived from "religare" [to bind together], wherefore Augustine says (De Vera Relig. 55): "May religion bind us to the one Almighty God." However, whether religion take its name from frequent reading, or from a repeated choice of what has been lost through negligence, or from being a bond, it denotes properly a relation to God. For it is He to Whom we ought to be bound as to our unfailing principle; to Whom also our choice should be resolutely directed as to our last end; and Whom we lose when we neglect Him by sin, and should recover by believing in Him and confessing our faith.
Reply to Objection 1. Religion has two kinds of acts. Some are its proper and immediate acts, which it elicits, and by which man is directed to God alone, for instance, sacrifice, adoration and the like. But it has other acts, which it produces through the medium of the virtues which it commands, directing them to the honor of God, because the virtue which is concerned with the end, commands the virtues which are concerned with the means. Accordingly "to visit the fatherless and widows in their tribulation" is an act of religion as commanding, and an act of mercy as eliciting; and "to keep oneself unspotted from this world" is an act of religion as commanding, but of temperance or of some similar virtue as eliciting.
Reply to Objection 2. Religion is referred to those things one exhibits to one's human kindred, if we take the term religion in a broad sense, but not if we take it in its proper sense. Hence, shortly before the passage quoted, Augustine says: "In a stricter sense religion seems to denote, not any kind of worship, but the worship of God."
Reply to Objection 3. Since servant implies relation to a lord, wherever there is a special kind of lordship there must needs be a special kind of service. Now it is evident that lordship belongs to God in a special and singular way, because He made all things, and has supreme dominion over all. Consequently a special kind of service is due to Him, which is known as "latria" in Greek; and therefore it belongs to religion.
Reply to Objection 4. We are said to worship those whom we honor, and to cultivate [In the Latin the same word 'colere' stands for 'worship' and 'cultivate']: a man's memory or presence: we even speak of cultivating things that are beneath us, thus a farmer [agricola] is one who cultivates the land, and an inhabitant [incola] is one who cultivates the place where he dwells. Since, however, special honor is due to God as the first principle of all things, to Him also is due a special kind of worship, which in Greek is Eusebeia or Theosebeia, as Augustine states (De Civ. Dei x, 1).
Reply to Objection 5. Although the name "religious" may be given to all in general who worship God, yet in a special way religious are those who consecrate their whole life to the Divine worship, by withdrawing from human affairs. Thus also the term "contemplative" is applied, not to those who contemplate, but to those who give up their whole lives to contemplation. Such men subject themselves to man, not for man's sake but for God's sake, according to the word of the Apostle (Galatians 4:14), "You... received me as an angel of God, even as Christ Jesus."
Source: Summa Theologica (New Advent)