Question Ten of Chapter Nine of the Third Decade
Whether it is lawful to keep relics of saints with oneself
It is asked whether it is lawful to carry relics of saints and keep them in any manner whatsoever.
It is answered that the same reasoning applies to relics of saints as to the Gospel or divine words: if such things are carried from confidence in God and in the saints whose relics they are, it is not unlawful.
But if some other vain thing were attended to in this connection—for example, that the vessel be triangular, or anything of this kind which pertains to irreverence toward God and the saints—it is superstitious and unlawful, as Thomas says in the place cited immediately above, in the reply to the third argument.