3 Therefore, in order to preserve the Italian clergy from the pernicious influences of the times, we deem it opportune, venerable brethren, to recall in this our letter, the true and invariable principles that should regulate ecclesiastical education and the entire sacred ministry. The Catholic priesthood-divine in its origin, supernatural in its essence, immutable in its character, is not an institution that can accommodate itself with ease to human systems and opinions. A participation of the eternal priesthood of Jesus Christ, it must perpetuate even to the consummation of ages the same mission that the Eternal Father confided to His Incarnate Word: "Sicut misit me Pater, et ego mitto vos."(1) To work the eternal salvation of souls will always be the great commandment of which it must never fall short, as to faithfully fulfil it, it must never cease to have recourse to those supernatural aids and those divine rules of thought and of action which Jesus Christ gave His Apostles when He sent them throughout the whole world to convert the nations to the Gospel. Therefore St. Paul in his letters reminds us that the priest can never be anything but the legate, the minister of Christ, the dispenser of His mysteries,(2) and he represents him to us as dwelling in a high place,(3) as a mediator between heaven and earth, to treat with God, about the supreme interests of the human race, which are those of everlasting life. The idea that holy books give us of the Christian priesthood, is that it is a supernatural institution superior to all those of earth, and as far separated from them as the divine is from the human.
Source: Fin dal Principio (Vatican.va)