Objection 1. It would seem that the punishment of sacrilege should not be pecuniary. A pecuniary punishment is not wont to be inflicted for a criminal fault. But sacrilege is a criminal fault, wherefore it is punished by capital sentence according to civil law [Dig. xlviii, 13; Cod. i, 3, de Episc. et Cleric.]. Therefore sacrilege should not be awarded a pecuniary punishment.
Objection 2. Further, the same sin should not receive a double punishment, according to Nahum 1:9, "There shall not rise a double affliction." But sacrilege is punished with excommunication; major excommunication, for violating a sacred person, and for burning or destroying a church, and minor excommunication for other sacrileges. Therefore sacrilege should not be awarded a pecuniary punishment.
Objection 3. Further, the Apostle says (1 Thessalonians 2:5): "Neither have we taken an occasion of covetousness." But it seems to involve an occasion of covetousness that a pecuniary punishment should be exacted for the violation of a sacred thing. Therefore this does not seem to be a fitting punishment of sacrilege.
On the contrary, It is written [XVII, qu. iv, can. Si quis contumax]: "If anyone contumaciously or arrogantly take away by force an escaped slave from the confines of a church he shall pay nine hundred soldi": and again further on (XVII, qu. iv, can. Quisquis inventus, can. 21): "Whoever is found guilty of sacrilege shall pay thirty pounds of tried purest silver."
I answer that, In the award of punishments two points must be considered. First equality, in order that the punishment may be just, and that "by what things a man sinneth by the same... he may be tormented" (Wisdom 11:17). On this respect the fitting punishment of one guilty of sacrilege, since he has done an injury to a sacred thing, is excommunication [Append. Gratian. on can. Si quis contumax, quoted above] whereby sacred things are withheld from him. The second point to be considered is utility. For punishments are inflicted as medicines, that men being deterred thereby may desist from sin. Now it would seem that the sacrilegious man, who reverences not sacred things, is not sufficiently deterred from sinning by sacred things being withheld from him, since he has no care for them. Wherefore according to human laws he is sentenced to capital punishment, and according to the statutes of the Church, which does not inflict the death of the body, a pecuniary punishment is inflicted, in order that men may be deterred from sacrilege, at least by temporal punishments.
Reply to Objection 1. The Church inflicts not the death of the body, but excommunication in its stead.
Reply to Objection 2. When one punishment is not sufficient to deter a man from sin, a double punishment must be inflicted. Wherefore it was necessary to inflict some kind of temporal punishment in addition to the punishment of excommunication, in order to coerce those who despise spiritual things.
Reply to Objection 3. If money were exacted without a reasonable cause, this would seem to involve an occasion of covetousness. But when it is exacted for the purpose of man's correction, it has a manifest utility, and consequently involves no occasion of avarice.
Source: Summa Theologica (New Advent)