Objection 1. It would seem that sacrilege is not the violation of a sacred thing. It is stated (XVII, qu. iv [Append. Gratian, on can. Si quis suadente]): "They are guilty of sacrilege who disagree about the sovereign's decision, and doubt whether the person chosen by the sovereign be worthy of honor." Now this seems to have no connection with anything sacred. Therefore sacrilege does not denote the violation of something sacred.
Objection 2. Further, it is stated further on [Append. Gratian, on can. Constituit.] that if any man shall allow the Jews to hold public offices, "he must be excommunicated as being guilty of sacrilege." Yet public offices have nothing to do with anything sacred. Therefore it seems that sacrilege does not denote the violation of a sacred thing.
Objection 3. Further, God's power is greater than man's. Now sacred things receive their sacred character from God. Therefore they cannot be violated by man: and so a sacrilege would not seem to be the violation of a sacred thing.
On the contrary, Isidore says (Etym. x) that "a man is said to be sacrilegious because he selects," i.e. steals, "sacred things."
I answer that, As stated above (81, 5; I-II, 101, 4), a thing is called "sacred" through being deputed to the divine worship. Now just as a thing acquires an aspect of good through being deputed to a good end, so does a thing assume a divine character through being deputed to the divine worship, and thus a certain reverence is due to it, which reverence is referred to God. Therefore whatever pertains to irreverence for sacred things is an injury to God, and comes under the head of sacrilege.
Reply to Objection 1. According to the Philosopher (Ethic. i, 2) the common good of the nation is a divine thing, wherefore in olden times the rulers of a commonwealth were called divines, as being the ministers of divine providence, according to Wisdom 6:5, "Being ministers of His kingdom, you have not judged rightly." Hence by an extension of the term, whatever savors of irreverence for the sovereign, such as disputing his judgment, and questioning whether one ought to follow it, is called sacrilege by a kind of likeness.
Reply to Objection 2. Christians are sanctified by faith and the sacraments of Christ, according to 1 Corinthians 6:11, "But you are washed, but you are sanctified." Wherefore it is written (1 Peter 2:9): "You are a chosen generation, a kingly priesthood, a holy nation, a purchased people." Therefore any injury inflicted on the Christian people, for instance that unbelievers should be put in authority over it, is an irreverence for a sacred thing, and is reasonably called a sacrilege.
Reply to Objection 3. Violation here means any kind of irreverence or dishonor. Now as "honor is in the person who honors and not in the one who is honored" (Ethic. i, 5), so again irreverence is in the person who behaves irreverently even though he do no harm to the object of his irreverence. Hence, so far he is concerned, he violates the sacred thing, though the latter be not violated in itself.
Source: Summa Theologica (New Advent)