Objection 1. It would seem that religion has not an external act. It is written (John 4:24): "God is a spirit, and they that adore Him, must adore Him in spirit and in truth." Now external acts pertain, not to the spirit but to the body. Therefore religion, to which adoration belongs, has acts that are not external but internal.
Objection 2. Further, the end of religion is to pay God reverence and honor. Now it would savor of irreverence towards a superior, if one were to offer him that which properly belongs to his inferior. Since then whatever man offers by bodily actions, seems to be directed properly to the relief of human needs, or to the reverence of inferior creatures, it would seem unbecoming to employ them in showing reverence to God.
Objection 3. Further, Augustine (De Civ. Dei vi, 10) commends Seneca for finding fault with those who offered to idols those things that are wont to be offered to men, because, to wit, that which befits mortals is unbecoming to immortals. But such things are much less becoming to the true God, Who is "exalted above all gods" [Psalm 94:3. Therefore it would seem wrong to worship God with bodily actions. Therefore religion has no bodily actions.
On the contrary, It is written (Psalm 83:3): "My heart and my flesh have rejoiced in the living God." Now just as internal actions belong to the heart, so do external actions belong to the members of the flesh. Therefore it seems that God ought to be worshiped not only by internal but also by external actions.
I answer that, We pay God honor and reverence, not for His sake (because He is of Himself full of glory to which no creature can add anything), but for our own sake, because by the very fact that we revere and honor God, our mind is subjected to Him; wherein its perfection consists, since a thing is perfected by being subjected to its superior, for instance the body is perfected by being quickened by the soul, and the air by being enlightened by the sun. Now the human mind, in order to be united to God, needs to be guided by the sensible world, since "invisible things... are clearly seen, being understood by the things that are made," as the Apostle says (Romans 1:20). Wherefore in the Divine worship it is necessary to make use of corporeal things, that man's mind may be aroused thereby, as by signs, to the spiritual acts by means of which he is united to God. Therefore the internal acts of religion take precedence of the others and belong to religion essentially, while its external acts are secondary, and subordinate to the internal acts.
Reply to Objection 1. Our Lord is speaking of that which is most important and directly intended in the worship of God.
Reply to Objection 2. These external things are offered to God, not as though He stood in need of them, according to Psalm 49:13, "Shall I eat the flesh of bullocks? or shall I drink the blood of goats?" but as signs of the internal and spiritual works, which are of themselves acceptable to God. Hence Augustine says (De Civ. Dei x, 5): "The visible sacrifice is the sacrament or sacred sign of the invisible sacrifice."
Reply to Objection 3. Idolaters are ridiculed for offering to idols things pertaining to men, not as signs arousing them to certain spiritual things, but as though they were of themselves acceptable to the idols; and still more because they were foolish and wicked.
Source: Summa Theologica (New Advent)