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.
St. Teresa of
"One night, too, about this time, I thought the devils were stifling me; and when the nuns had sprinkled a great deal of holy water about I saw a huge crowd of them running away as quickly as though they were about to fling themselves down a steep place."
"I will only describe something that happened to me one night of All Souls. I was in an oratory: I had said one nocturnes and was repeating some very devotional prayers which follow it -- they are extremely devotional: we have them in our office-book -- when actually the devil himself alighted on the book, to prevent me from finishing the prayer. I made the sign of the Cross and he went away. I then began again and he came back. I think I began that prayer three times and not until I had sprinkled some holy water on him could I finish it. At the same moment I saw several souls coming out of purgatory: their time there must have been nearly up and I thought that perhaps the devil was trying to impede their deliverance."
Keep your soul beautifully pure in God's sight by making the Sign of the Cross carefully while saying, "By this holy water and by Thy Precious Blood wash away all my sins, O Lord."