Roman Catechism 21.51

Chapter 21 — Part II: The Sacraments — Holy Orders

Section 51. Hence we read in the Apocalypse: Christ hath washed us front our sins. in his own blood and hath made us a kingdom, and priests to God and his Father. In like manner was it said by the Prince of the Apostles: Be you also as living stones built up, a spiritual house a holy priesthood offering up spiritual sacrifices acceptable to God by Jesus Christ; while the Apostle exhorts us to present our bodies a living sacrifice holy , pleasing unto God your reasonable service. And long before this David had said: A sacrifice to God is an afflicted spirit: a contrite and humble heart O God thou wilt not despise. All this clearly regards the internal priesthood.