The Church has instituted the sacramentals, those rites or objects on which a particular blessing bestows a special virtue for preservation against the influence of the devil. Among the sacramentals, St. Teresa favored the use of holy water:
From long experience, I have learned that there is nothing like holy water to put devils to flight and to prevent them from coming back again. They also flee from the Cross, but return; so holy water must have great virtue. For my own part, whenever I take it, my soul feels a particular and most notable consolation. In fact, it is quite usual for me to be conscious of a refreshment which I cannot possibly describe, resembling an inward joy which comforts my whole soul. This is not fancy, or something which has happened to me only once; it has happened again and again and I have observed it most attentively [Life, xxxi, Peers, I, 205].
...The Church, in the different prayers for the blessing of water, asks insistently that power be given to this water "to put to flight all power of the enemy, to expel this enemy with all the rebel angels, to drive it away, to destroy the influences of the evil spirit and to cast out the venomous serpent.
"I often reflect on the great importance of everything ordained by the Church," comments St. Teresa, "and it makes me very happy to find that those words of the Church are so powerful that they impart their power to the water and make it so very different from water which has not been blessed."
Source: I Want To See God, A Practical Synthesis of Carmelite Sprituality, by Pere Marie-Eugene, OCD, pps. 115-117.