Objection 1. It would seem that religion is not a virtue. Seemingly it belongs to religion to pay reverence to God. But reverence is an act of fear which is a gift, as stated above (Question 19, Article 09). Therefore religion is not a virtue but a gift
Objection 2. Further, every virtue is a free exercise of the will, wherefore it is described as an "elective" or voluntary "habit" [Ethic. ii, 6. Now, as stated above (1, ad 3) "latria" belongs to religion, and "latria" denotes a kind of servitude. Therefore religion is not a virtue.
Objection 3. Further, according to Ethic. ii, 1, aptitude for virtue is in us by nature, wherefore things pertaining to virtue belong to the dictate of natural reason. Now, it belongs to religion "to offer ceremonial worship to the Godhead" [Cf. 1, and ceremonial matters, as stated above (I-II, 99, 3, ad 2; I-II, 101), do not belong to the dictate of natural reason. Therefore religion is not a virtue.
On the contrary, It is enumerated with the other virtues, as appears from what has been said above (Article 80).
I answer that, As stated above (58, 3; I-II, 55, 3,4) "a virtue is that which makes its possessor good, and his act good likewise," wherefore we must needs say that every good act belongs to a virtue. Now it is evident that to render anyone his due has the aspect of good, since by rendering a person his due, one becomes suitably proportioned to him, through being ordered to him in a becoming manner.
But order comes under the aspect of good, just as mode and species, according to Augustine (De Nat. Boni iii). Since then it belongs to religion to pay due honor to someone, namely, to God, it is evident that religion is a virtue.
Reply to Objection 1. To pay reverence to God is an act of the gift of fear. Now it belongs to religion to do certain things through reverence for God. Hence it follows, not that religion is the same as the gift of fear, but that it is referred thereto as to something more excellent; for the gifts are more excellent than the moral virtues, as stated above (9, 1, ad 3; I-II, 68, 8).
Reply to Objection 2. Even a slave can voluntarily do his duty by his master, and so "he makes a virtue of necessity" [Jerome, Ep. liv, ad Furiam.], by doing his duty voluntarily. On like manner, to render due service to God may be an act of virtue, in so far as man does so voluntarily.
Reply to Objection 3. It belongs to the dictate of natural reason that man should do something through reverence for God. But that he should do this or that determinate thing does not belong to the dictate of natural reason, but is established by Divine or human law.
Source: Summa Theologica (New Advent)