Objection 1. It would seem that idolatry is not a sin. Nothing is a sin that the true faith employs in worshipping God. Now the true faith employs images for the divine worship: since both in the Tabernacle were there images of the cherubim, as related in Exodus 25, and in the Church are images set up which the faithful worship. Therefore idolatry, whereby idols are worshipped, is not a sin.
Objection 2. Further, reverence should be paid to every superior. But the angels and the souls of the blessed are our superiors. Therefore it will be no sin to pay them reverence by worship, of sacrifices or the like.
Objection 3. Further, the most high God should be honored with an inward worship, according to John 4:24, "God... they must adore... in spirit and in truth": and Augustine says (Enchiridion iii), that "God is worshipped by faith, hope and charity." Now a man may happen to worship idols outwardly, and yet not wander from the true faith inwardly. Therefore it seems that we may worship idols outwardly without prejudice to the divine worship.
On the contrary, It is written (Exodus 20:5): "Thou shalt not adore them," i.e. outwardly, "nor serve them," i.e. inwardly, as a gloss explains it: and it is a question of graven things and images. Therefore it is a sin to worship idols whether outwardly or inwardly.
I answer that, There has been a twofold error in this matter. For some [The School of Plato] have thought that to offer sacrifices and other things pertaining to latria, not only to God but also to the others aforesaid, is due and good in itself, since they held that divine honor should be paid to every superior nature, as being nearer to God. But this is unreasonable. For though we ought to revere all superiors, yet the same reverence is not due to them all: and something special is due to the most high God Who excels all in a singular manner: and this is the worship of latria.
Nor can it be said, as some have maintained, that "these visible sacrifices are fitting with regard to other gods, and that to the most high God, as being better than those others, better sacrifices, namely, the service of a pure mind, should be offered" [Augustine, as quoted below]. The reason is that, as Augustine says (De Civ. Dei x, 19), "external sacrifices are signs of internal, just as audible words are signs of things. Wherefore, just as by prayer and praise we utter significant words to Him, and offer to Him in our hearts the things they signify, so too in our sacrifices we ought to realize that we should offer a visible sacrifice to no other than to Him Whose invisible sacrifice we ourselves should be in our hearts."
Others held that the outward worship of latria should be given to idols, not as though it were something good or fitting in itself, but as being in harmony with the general custom. Thus Augustine (De Civ. Dei vi, 10) quotes Seneca as saying: "We shall adore," says he, "in such a way as to remember that our worship is in accordance with custom rather than with the reality": and (De Vera Relig. v) Augustine says that "we must not seek religion from the philosophers, who accepted the same things for sacred, as did the people; and gave utterance in the schools to various and contrary opinions about the nature of their gods, and the sovereign good." This error was embraced also by certain heretics [The Helcesaitae], who affirmed that it is not wrong for one who is seized in time of persecution to worship idols outwardly so long as he keeps the faith in his heart.
But this is evidently false. For since outward worship is a sign of the inward worship, just as it is a wicked lie to affirm the contrary of what one holds inwardly of the true faith so too is it a wicked falsehood to pay outward worship to anything counter to the sentiments of one's heart. Wherefore Augustine condemns Seneca (De Civ. Dei vi, 10) in that "his worship of idols was so much the more infamous forasmuch as the things he did dishonestly were so done by him that the people believed him to act honestly."
Reply to Objection 1. Neither in the Tabernacle or Temple of the Old Law, nor again now in the Church are images set up that the worship of latria may be paid to them, but for the purpose of signification, in order that belief in the excellence of angels and saints may be impressed and confirmed in the mind of man. It is different with the image of Christ, to which latria is due on account of His Divinity, as we shall state in the III, 25, 3.
The Replies to the Second and Third Objections are evident from what has been said above.
Source: Summa Theologica (New Advent)