Purification of Priests

Who will endure the day of his coming? and who can stand when he appears? For he is like the refiner's fire, or like the fuller's lye. He will sit refining and purifying silver, and he will purify the sons of Levi, refining them like gold or like silver that they may offer due sacrifice to the Lord. Then the sacrifice of Judah and Jerusalem will please the Lord, as in the days of old, as in years gone by. (Malachi 3:1-4)

"The Lord evidently wishes his priests to be pure and holy; that being cleansed from every defect, they may approach the altar to offer sacrifice to him...In the Book of Leviticus he says: They shall be holy to their God, and shall not profane his name; for they offer the burnt offerings of the Lord, and the bread of their God, and therefore they shall be holy. The priests of the Old Law, then, were commanded to be holy, because they offered to God incense and the loaves of the proposition, that were but a figure of the most holy Sacrament of the Altar. How much greater should the purity and sanctity of the priests of the New Law that offer to God the immaculate Lamb, his own very Son? Estius says that we do not offer calves or incense, as the priests of the Old Law did, 'but the Body of Christ who is hanging on the Cross.'" (St. Alphonsus Liguori, Dignity and Duties of the Priest, 56)