27 Now, in order that all these recommendations should have the results We hope for, you know, Venerable Brethren, how necessary is the prudent and assiduous work of those whom Christ our Lord sends as "labourers into His harvest," that is to say the clergy. Remember, therefore, that your chief care must be to foster in the holiness which becomes them the clergy you already possess, and worthily to form your ecclesiastical students for so sacred an office by the very best available education and training. And although your carefulness in this respect calls for no stimulus, nevertheless We exhort and even implore you to give the matter your most careful attention. Nothing can be of greater importance for the good of the Church; but as Our Predecessors of happy memory, Leo XIII and Pius X, have definitely written on this subject, there is no need of further counsels from Us. We only beg of you that the writings of those wise pontiffs, and especially Pius X's "Exhortation to the Clergy," should, thanks to your insistent admonitions, not be forgotten, but ever attended to carefully.
Source: Ad Beatissimi Apostolorum (Vatican.va)