By this holy water and by Your Precious Blood, wash away all my sins, O Lord.
St. Theresa of Avila on holy water: "From long experience I have learned that there is nothing like holy water to put devils to flight and prevent them from coming back again. They also flee from the cross, but return; so holy water must have great value."
Holy water is a means of spiritual wealth -- a sacramental that remits venial sin. The Church strongly urges its use, especially when dangers threaten. The devil hates holy water because of its power over him. He cannot long abide in a place or near a person that is often sprinkled with this blessed water.
Also see Holy Water