8 Thus, indeed, Venerable Brethren, you may know and see why and in what way the younger members of the Church ought to be instructed in higher doctrines that they perform their duties with ease and utility at the present time. That these may be thoroughly grounded and accomplished in the study of humanities they should not enter upon the study of sacred theology before having undergone a preparation in philosophy. We mean that deep and real philosophy, the investigator of the loftiest problems, the best patron of truth: by virtue of which they themselves will not be tossed about nor carried away "by every wind of doctrine, by the wickedness of men, by the craftiness by which they lie in wait to deceive" (Ephes. iv. 14), and will enable them to give to other doctrines the aid of truth, by the discussion and refutation of captious and deceptive theories. With this object we have already advised that the works of the great Aquinas should be in their hands, and should be constantly and carefully explained; and We have often laid stress upon the same thing with solemn words; and We believe that the best fruits are thence received by the clergy, and We shall confidently look for fruits still more excellent and abundant. Indeed, the method of the Angelic Doctor is admirably adapted for training minds, wonderfully fitted for use in making comments, in philosophising, in discoursing forcibly and incontrovertibly: for it shows clearly each subject connected one with another in a continuous series, all however joined together and fitting into each other, all leading to the highest principles; then it raises one to the contemplation of God, Who is the efficient cause and strength and highest type of all things, to Whom finally all philosophy and man himself, such as he is, ought to be referred. Thus truly the knowledge of things are held together, as they are admirably shown, so also are they most firmly established by St. Thomas; by conflict with which knowledge, as the ancient sects of errors have entirely disappeared, so the new, unlike them rather in name and kind than in fact, as soon as they have put forth their heads fall, laid low by the same blows, as indeed many of Our writers have shown. Truly human reason desires to penetrate freely into the hidden and secret knowledge of things, nor can it do otherwise, but with Aquinas for Our author and master it does this more quickly and freely because it does it safely without any danger of passing over the boundaries of truth. For neither can you rightly call that liberty which gathers and scatters opinions according to its own will and pleasure, nay rather it is to be reputed the vilest license, lying, and false science, a disgrace and slavery of the mind. He indeed is the true Doctor who walks within the confines of truth, who not only never differs from God, the Head and Fount of all truth, but is always strictly in accordance with Him and always follows Him when disclosing His secrets in any manner; who no less piously listens to the Roman Pontiff when speaking, reverses in him the divine authority and fully holds that "submission to the Roman Pontiff is necessary to salvation." ( Opusc. contra errores Groecoram .) In his school, therefore, let the cleric be brought up and exercised both in philosophy and theology: for he will then be learned and strong as the mightiest to fight the sacred combats.
Source: Officio Sanctissimo (Vatican.va)