This ceremony of sprinkling with holy water is observed, chiefly by the Roman Catholic Church, when consecrating churches or purifying houses, as an aid to unction and immediately before the principal Mass. Sprinkling the congregation at Mass was ordered by Pope Leo IV in 847, but the practice is attested in the eighth century. The words of Psalm 51:7 usually accompany the ceremony, although the original objective of the psalmist was inward purification by the merciful Lord.