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.—EWTN, 4th Week of Lent.
Q. 1071. What is holy water?
A. Holy water is water blessed by the priest with solemn prayer to beg God's blessing on those who use it, and protection from the powers of darkness.
Q. 1052. What is a sacramental?
A. A sacramental is anything set apart or blessed by the Church to excite good thoughts and to increase devotion, and through these movements of the heart to remit venial sin.
Q. 1053. How do the Sacramentals excite good thoughts and increase devotion?
A. The Sacramentals excite good thoughts by recalling to our minds some special reason for doing good and avoiding evil; especially by reminding us of some holy person, event or thing through which blessings have come to us. They increase devotion by fixing our minds on particular virtues and by helping us to understand and desire them.
Q. 1054. Do the Sacramentals of themselves remit venial sins?The Catechism of the Catholic Church
A. The Sacramentals of themselves do not remit venial sins, but they move us to truer devotion, to greater love for God and greater sorrow for our sins, and this devotion, love and sorrow bring us grace, and the grace remits venial sins.
CCC1677 Sacramentals are sacred signs instituted by the Church. They prepare men to receive the fruit of the sacraments and sanctify different circumstances of life.Using Holy Water before and after Mass.
With the Sign of the Cross one might also add one of the following phrases:
By this holy water and by Your Precious Blood, wash away all my sins, O Lord.
or [Psalm 51 (50), Asperges Me from the Mass]
Purify me with hyssop, Lord, and I shall be clean of sin.
Wash me, and I shall be whiter than snow.
Aspérges me, Dómine, hyssópo et mundábor,
Lavábis me, et super nivem dealbábor.
or [Vidi Aquam from the Mass, Ezekiel 47]
I saw water flowing from the right side of the temple, alleluia; and all they to whom that water came were saved, and they shall say, alleluia, alleluia.
Vidi aquam egrediéntem de templo, a látere dextro, allelúia: et omnes ad quos pervénit aqua ista salvi facti sunt et dicent: allelúia, allelúia.