33 Nevertheless, it is quite true that so holy an office demands holiness in him who holds it. A priest should have a loftiness of spirit, a purity of heart and a sanctity of life befitting the solemnity and holiness of the office he holds. For this, as We have said, makes the priest a mediator between God and man; a mediator in the place, and by the command of Him who is "the one mediator of God and men, the man Jesus Christ." The priest must, therefore, approach as close as possible to the perfection of Him whose vicar he is, and render himself ever more and more pleasing to God, by the sanctity of his life and of his deeds; because more than the scent of incense, or the beauty of churches and altars, God loves and accepts holiness. "They who are the intermediaries between God and His people," says St. Thomas, "must bear a good conscience before God, and a good name among men." On the contrary, whosoever handles and administers holy things, while blameworthy in his life, profanes them and is guilty of sacrilege: "They who are not holy ought not to handle holy things."
Source: Ad Catholici Sacerdotii (Vatican.va)