The following is a selection from the encyclical Fin Dal Principio by Pope Leo XIII, in which he speaks of the education of the clergy. I think this offers some very valuable insights with regard to the adaptability of the priestly office to modern times. Too often it is said today that the Church must change with the times, so as to conform itself to the modern world. This is often what is meant by the use of the term "aggiornamento," which came into use under Pope John XIII. The false "aggiornamento" so often praised today is one that looks with scorn upon the detachment of the Church from the world. It is absolutely crucial that one's notion of "aggiornamento" not become distorted, so as to actually bring about a change in the most important parts of religion. This includes the liturgy of the Church. The priesthood is especially connected to the liturgy, so I think that Pope Leo's words here are quite valuable.
Venerable Brethren, Health and Apostolic Benediction.
1. From the beginning of our Pontificate having gravely considered the serious conditions of society, we are not slow to recognize, as one of the most urgent duties of the Apostolic office, that of devoting a most special care to the education of the clergy.
2. We see in fact that all our designs to bring about a restoration of Christian life among our people, would be in vain if in the ecclesiastical state the sacerdotal spirit was not preserved intact and vigorous. This we have not ceased to do, as far as was possible to us, both with institutions and writings directed to that end. And now a particular solicitude regarding the clergy of Italy moves us, venerable brethren, again to treat on this subject of so great importance. It is true, beautiful and continued testimonies have been shown of learning, piety and zeal, among which we are glad to praise the alacrity with which, seconding the impulse and direction of their bishops, they cooperate in that Catholic movement which we have so much at heart. We cannot altogether, however, hide the preoccupation of our soul at seeing for some time past a certain desire of innovation insinuating itself here and there, as regards the constitution as well as the multiform actions of the sacred ministry. Now it is easy to foresee the grave consequences which we should have to deplore if a speedy remedy were not applied to this innovating tendency.
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."' 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, and he represents him to us as dwelling in a high place  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.
4. This same high idea is clearly brought out by the works of the Fathers, the laws of the Roman Pontiffs, and the Bishops, by the decrees of the Councils, and by the unanimous teaching of the Doctors and of the Catholic schools. Above all, the tradition of the Church with one voice proclaims that the priest is another Christ, and that the priesthood though exercised on earth merits to be numbered among the orders of heaven; because it is given to them to administer things that are wholly celestial and upon them is conferred a power that God has not trusted even to the angels; a power and ministry which regard the government of souls, and which is the art of arts. Therefore, education, studies, customs, and whatever comprises the sacerdotal discipline have always been considered by the Church as belonging entirely to herself, not merely distinct, but altogether separate from the ordinary rules of secular life. This distinction and separation must, therefore, remain unaltered, even in our own times, and any tendency to accommodate or confound the ecclesiastical life and education with the secular life and education must be considered as reproved, not only by the traditions of Christian ages, but by the apostolic doctrine itself and the ordinances of Jesus Christ.
5. Certainly in the formation of the clergy and the sacerdotal ministry, it is reasonable that regard should be had to the varied conditions of the times. Therefore we are far from rejecting the idea of such changes as would render the work of the clergy still more efficacious in the society in which they live, and it is for that reason that it has seemed necessary to us to promote among them a more solid and finished culture, and to open a still wider field to their ministry; but every other innovation which could in any way prejudice what is essential to the priest must be regarded as altogether blameworthy. The priest is above all constituted master, physician and shepherd of souls, and a guide to an end not enclosed within the bounds of this present life. Now he can never fully correspond if he is not well versed in the science of divine and sacred things, if he is not furnished with that piety which makes a man of God; and if he does not take every care to render his teachings valuable by the efficacy of his example, conformably to the admonition given to the sacred pastor by the Prince of the Apostles: "Forma facti gregis ex animo." For those who watch the times and the changeable condition of society, these are the right and the greatest gifts that could shine in the Catholic priest, together with the principles of faith; every other quality natural and human would certainly be commendable, but would not have with regard to the sacerdotal office anything but secondary and relative importance. If, therefore, it is reasonable and just that the clergy should accommodate themselves as far as is permitted to the needs of the present age, it is still more necessary that the present depravity of the century should not be yielded to, but strongly resisted; and this while corresponding naturally to the high end of the priesthood, will also render their ministry still more fruitful by increasing its dignity, and therefore gaining it respect. It is seen everywhere how the spirit of naturalism tends to penetrate every part of the social body, even the most healthy; a spirit which fills the minds with pride and causes them to rebel against every authority; depraves the heart and turns it after the desire of earthly goods, neglecting those eternal.
6. It is greatly to be feared that some influence of this spirit, so evil, and already so widely diffused, might insinuate itself even among ecclesiastics, particularly among those of less experience. What sad effects would not arise if that gravity of conduct which belongs to the priest, should be in any way lessened; if he should yield with lightness to the charm of every novelty; if he should deport himself with pretentious indocility towards his superiors; if he should lose that weight and measure in discussion which is so necessary, particularly in matters of faith and morals.
1. Jn 20:21.
2. 2 Cor 5: 20; 6: 4; I Cor 4: 1.
3. Heb 5: 1.
4. John Chrysostom, De sacerdotio III, n. 4.
5. Ibid. n. 5.
6. Gregory the Great, Regula pastoralis, pars 1, cap. 1.
7. I Pt 5: i.