Objection 1. It would seem that devotion is not an act of religion. Devotion, as stated above (Article 1), consists in giving oneself up to God. But this is done chiefly by charity, since according to Dionysius (Div. Nom. iv) "the Divine love produces ecstasy, for it takes the lover away from himself and gives him to the beloved." Therefore devotion is an act of charity rather than of religion.
Objection 2. Further, charity precedes religion; and devotion seems to precede charity; since, in the Scriptures, charity is represented by fire, while devotion is signified by fatness which is the material of fire [Canticles 8:6; Psalm 52:6. Therefore devotion is not an act of religion.
Objection 3. Further, by religion man is directed to God alone, as stated above (Question 81, Article 1). But devotion is directed also to men; for we speak of people being devout to certain holy men, and subjects are said to be devoted to their masters; thus Pope Leo says [Serm. viii, De Pass. Dom.] that the Jews "out of devotion to the Roman laws," said: "We have no king but Caesar." Therefore devotion is not an act of religion.
On the contrary, Devotion is derived from "devovere," as stated (1).
But a vow is an act of religion. Therefore devotion is also an act of religion.
I answer that, It belongs to the same virtue, to will to do something, and to have the will ready to do it, because both acts have the same object. For this reason the Philosopher says (Ethic. v, 1): "It is justice whereby men both will end do just actions." Now it is evident that to do what pertains to the worship or service of God, belongs properly to religion, as stated above (Article 81). Wherefore it belongs to that virtue to have the will ready to do such things, and this is to be devout. Hence it is evident that devotion is an act of religion.
Reply to Objection 1. It belongs immediately to charity that man should give himself to God, adhering to Him by a union of the spirit; but it belongs immediately to religion, and, through the medium of religion, to charity which is the principle of religion, that man should give himself to God for certain works of Divine worship.
Reply to Objection 2. Bodily fatness is produced by the natural heat in the process of digestion, and at the same time the natural heat thrives, as it were, on this fatness. On like manner charity both causes devotion (inasmuch as love makes one ready to serve one's friend) and feeds on devotion. Even so all friendship is safeguarded and increased by the practice and consideration of friendly deeds.
Reply to Objection 3. Devotion to God's holy ones, dead or living, does not terminate in them, but passes on to God, in so far as we honor God in His servants. But the devotion of subjects to their temporal masters is of another kind, just as service of a temporal master differs from the service of God.
Source: Summa Theologica (New Advent)