10 If the education of all youth in general contributes a great deal to the true welfare of the state, this is much more true of the education of those aiming at ordination. To this matter you must give special attention; it should occupy the greater portion of your vigils and labors, since the youths destined for orders are the hope and, as it were, the incomplete form of future priests. You surely know how much the reputation of the Church and the eternal salvation of her people depend on priests. - In the education of clerics, two elements are absolutely necessary: learning for the development of the mind and virtue for the perfection of the spirit. To the ordinary humanistic subjects in which youths are educated must be added Sacred and Canonical studies. Care must be taken that their content is sound and everywhere pure, in full harmony with the documents of the Church and eloquent, so that the priest may be able to exhort . . . even those who contradict. - Holiness of life, without which knowledge puffs up and does not edify, consists not only in good and honorable habits, but also in that group of sacerdotal virtues which makes good priests exemplars of Jesus Christ, the eternal High Priest. For this purpose there are sacred seminaries. You have some for youths preparing for the priesthood and others for the education of seminarians, all of them well-founded. Choose teachers and spiritual directors for these institutions thoughtfully. They should be men of sound doctrine and good morals, men to whom you can confidently entrust a matter of such great importance. Choose rectors and spiritual guides who are outstanding in prudence, counsel, and experience. The common life and discipline should be so arranged by your authority that not only will the students never offend against piety, but that there will be an abundance of all aids which nourish piety. The students should thus be encouraged to make daily progress in acquiring the sacerdotal virtues. Your industrious and diligent labors in the education of priests will bear much desirable fruit, making your episcopal office easier to administer and producing a richer profit for all.
Source: Quod Multum (Vatican.va)