What is holy water and why do Catholics use it?

Holy water is water blessed by a priest or deacon for sacred use. Catholics use it to recall their Baptism, seek God's protection, and sanctify persons and places. It is a sacramental — a sacred sign that disposes the faithful to receive grace and signifies effects obtained through the Church's intercession.

Water as a Sign of Life and Purification in Scripture

The blessing of water for ritual use thus stands within this long biblical tradition of water as an instrument of God's cleansing, life-giving action.