Objection 1. It would seem that it is not unlawful to wear divine words at the neck. Divine words are no less efficacious when written than when uttered. But it is lawful to utter sacred words for the purpose of producing certain effects; (for instance, in order to heal the sick), such as the "Our Father" or the "Hail Mary," or in any way whatever to call on the Lord's name, according to Mark 16:17-18, "In My name they shall cast out devils, they shall speak with new tongues, they shall take up serpents." Therefore it seems to be lawful to wear sacred words at one's neck, as a remedy for sickness or for any kind of distress.
Objection 2. Further, sacred words are no less efficacious on the human body than on the bodies of serpents and other animals. Now certain incantations are efficacious in checking serpents, or in healing certain other animals: wherefore it is written (Psalm 57:5): "Their madness is according to the likeness of a serpent, like the deaf asp that stoppeth her ears, which will not hear the voice of the charmers, nor of the wizard that charmeth wisely." Therefore it is lawful to wear sacred words as a remedy for men.
Objection 3. Further, God's word is no less holy than the relics of the saints; wherefore Augustine says (Lib. L. Hom. xxvi) that "God's word is of no less account than the Body of Christ." Now it is lawful for one to wear the relics of the saints at one's neck, or to carry them about one in any way for the purpose of self-protection. Therefore it is equally lawful to have recourse to the words of Holy Writ, whether uttered or written, for one's protection.
Objection 4. On the other hand, Chrysostom says (Hom. xliii in Matth.) [Cf. the Opus Imperfectum in Matthaeum, among St. Chrysostom's works, and falsely ascribed to him]: "Some wear round their necks a passage in writing from the Gospel. Yet is not the Gospel read in church and heard by all every day? How then, if it does a man no good to have the Gospels in his ears, will he find salvation by wearing them round his neck? Moreover, where is the power of the Gospel? In the shapes of the letters or in the understanding of the sense? If in the shapes, you do well to wear them round your neck; if in the understanding, you will then do better to bear them in your heart than to wear them round your neck."
I answer that, In every incantation or wearing of written words, two points seem to demand caution. The first is the thing said or written, because if it is connected with invocation of the demons it is clearly superstitious and unlawful. On like manner it seems that one should beware lest it contain strange words, for fear that they conceal something unlawful. Hence Chrysostom says [Cf. the Opus Imperfectum in Matthaeum, among St. Chrysostom's works, falsely ascribed to him] that "many now after the example of the Pharisees who enlarged their fringes, invent and write Hebrew names of angels, and fasten them to their persons. Such things seem fearsome to those who do not understand them." Again, one should take care lest it contain anything false, because in that case also the effect could not be ascribed to God, Who does not bear witness to a falsehood.
In the second place, one should beware lest besides the sacred words it contain something vain, for instance certain written characters, except the sign of the Cross; or if hope be placed in the manner of writing or fastening, or in any like vanity, having no connection with reverence for God, because this would be pronounced superstitious: otherwise, however, it is lawful. Hence it is written in the Decretals (XXVI, qu. v, cap. Non liceat Christianis): "In blending together medicinal herbs, it is not lawful to make use of observances or incantations, other than the divine symbol, or the Lord's Prayer, so as to give honor to none but God the Creator of all."
Reply to Objection 1. It is indeed lawful to pronounce divine words, or to invoke the divine name, if one do so with a mind to honor God alone, from Whom the result is expected: but it is unlawful if it be done in connection with any vain observance.
Reply to Objection 2. Even in the case of incantations of serpents or any animals whatever, if the mind attend exclusively to the sacred words and to the divine power, it will not be unlawful. Such like incantations, however, often include unlawful observances, and rely on the demons for their result, especially in the case of serpents, because the serpent was the first instrument employed by the devil in order to deceive man. Hence a gloss on the passage quoted says: "Note that Scripture does not commend everything whence it draws its comparisons, as in the case of the unjust judge who scarcely heard the widow's request."
Reply to Objection 3. The same applies to the wearing of relics, for if they be worn out of confidence in God, and in the saints whose relics they are, it will not be unlawful. But if account were taken in this matter of some vain circumstance (for instance that the casket be three-cornered, or the like, having no bearing on the reverence due to God and the saints), it would be superstitious and unlawful.
Reply to Objection 4. Chrysostom is speaking the case in which more attention is paid the written characters than to the understanding of the words.
Source: Summa Theologica (New Advent)