Objection 1. It would seem that superstition is not a vice contrary to religion. One contrary is not included in the definition of the other.
But religion is included in the definition of superstition: for the latter is defined as being "immoderate observance of religion," according to a gloss on Colossians 2:23, "Which things have indeed a show of wisdom in superstition." Therefore superstition is not a vice contrary to religion.
Objection 2. Further, Isidore says (Etym. x): "Cicero [De Natura Deorum ii, 28 states that the superstitious were so called because they spent the day in praying and offering sacrifices that their children might survive [superstites] them." But this may be done even in accordance with true religious worship. Therefore superstition is not a vice opposed to religion.
Objection 3. Further, superstition seems to denote an excess. But religion admits of no excess, since, as stated above (81, 5, ad 3), there is no possibility of rendering to God, by religion, the equal of what we owe Him. Therefore superstition is not a vice contrary to religion.
On the contrary, Augustine says (De Decem Chord. Serm. ix): "Thou strikest the first chord in the worship of one God, and the beast of superstition hath fallen." Now the worship of one God belongs to religion. Therefore superstition is contrary to religion.
I answer that, As stated above (Question 81, Article 5), religion is a moral virtue. Now every moral virtue observes a mean, as stated above (I-II, 64, 1). Therefore a twofold vice is opposed to a moral virtue. One by way of excess, the other by way of deficiency. Again, the mean of virtue may be exceeded, not only with regard to the circumstance called "how much," but also with regard to other circumstances: so that, in certain virtues such as magnanimity and magnificence; vice exceeds the mean of virtue, not through tending to something greater than the virtue, but possibly to something less, and yet it goes beyond the mean of virtue, through doing something to whom it ought not, or when it ought not, and in like manner as regards other circumstances, as the Philosopher shows (Ethic. iv, 1,2,3).
Accordingly superstition is a vice contrary to religion by excess, not that it offers more to the divine worship than true religion, but because it offers divine worship either to whom it ought not, or in a manner it ought not.
Reply to Objection 1. Just as we speak metaphorically of good among evil things--thus we speak of a good thief--so too sometimes the names of the virtues are employed by transposition in an evil sense. Thus prudence is sometimes used instead of cunning, according to Luke 16:8, "The children of this world are more prudent [Douay: 'wiser'] in their generation than the children of light." It is in this way that superstition is described as religion.
Reply to Objection 2. The etymology of a word differs from its meaning. For its etymology depends on what it is taken from for the purpose of signification: whereas its meaning depends on the thing to which it is applied for the purpose of signifying it. Now these things differ sometimes: for "lapis" [a stone] takes its name from hurting the foot [laedere pedem], but this is not its meaning, else iron, since it hurts the foot, would be a stone. On like manner it does not follow that "superstition" means that from which the word is derived.
Reply to Objection 3. Religion does not admit of excess, in respect of absolute quantity, but it does admit of excess in respect of proportionate quantity, in so far, to wit, as something may be done in divine worship that ought not to be done.
Source: Summa Theologica (New Advent)