Thomas Aquinas

“ There was no idolatry in the first age, owing to the recent remembrance of the creation of the world, so that man still retained in his mind the knowledge of one God. In the sixth age idolatry was banished by the doctrine and power of Christ, who triumphed over the devil. ”

“ Even as to the genus of the sin, the Manichean heresy is more grievous than the sin of other idolaters, because it is more derogatory to the divine honor, since they set up two gods in opposition to one another, and hold many vain and fabulous fancies about God. ”

“ But the cause of idolatry could not be on the part of man's nature, since rather does man's natural reason dictate that there is one God, and that divine worship should not be paid to the dead or to inanimate beings. ”

“ The saying of the Apostle that "an idol is nothing in the world" means that those images which were called idols, were not animated, or possessed of a divine power, as Hermes maintained, as though they were composed of spirit and body. ”

“ Therefore unbelief, despair and hatred of God, which are opposed to internal worship, are graver sins than idolatry, which is opposed to external worship.
Objection 2: Further, the more a sin is against God the more grievous it is. Now, seemingly, a man acts more directly against God by blaspheming, or denying the faith, than by giving God's worship to another, which pertains to idolatry. Therefore blasphemy and denial of the faith are more grievous sins than idolatry. ”

“ In the same sense we must understand the saying that "what is offered in sacrifice to idols is not anything," because by being thus sacrificed the sacrificial flesh acquired neither sanctification, as the Gentiles thought, nor uncleanness, as the Jews held.
Reply to Objection 4: It was owing to the general custom among the Gentiles of worshipping any kind of creature under the form of images that the term "idolatry" was used to signify any worship of a creature, even without the use of images.
Art. 2 - Whether idolatry is a sin? ”

“ Again, idolatry is stated to be the cause, beginning and end of all sin, because there is no kind of sin that idolatry does not produce at some time, either through leading expressly to that sin by causing it, or through being an occasion thereof, either as a beginning or as an end, in so far as certain sins were employed in the worship of idols ”

“ For some deemed certain men to have been gods, whom they worshipped in the images of those men: for instance, Jupiter, Mercury, and so forth. Others again deemed the whole world to be one god, not by reason of its material substance, but by reason of its soul, which they believed to be God, for they held God to be nothing else than a soul governing the world by movement and reason: even as a man is said to be wise in respect not of his body but of his soul. ”

“ 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. ”

“ 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, 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. ”

“ Secondly, because man takes a natural pleasure in representations, as the Philosopher observes (Poet. iv) , wherefore as soon as the uncultured man saw human images skillfully fashioned by the diligence of the craftsman, he gave them divine worship ”