Listers, in his motu proprio Doctoris Angelici, Pope Saint Pius X strongly exhorts Catholic schools to study St. Thomas Aquinas and his irreplaceable Summa Theologica. The letter comes in the context of correcting misgivings among Italy and the adjacent islands about their use – or lack their of – regarding the Angelic Doctor.

6 of the most potent quotes have been listed below, and the full text of the letter follows the quotes.

St. Thomas Aquinas, Angelic Doctor, Patron of All Catholic Schools, pray for us. 
Pope St. Pius X, pray for us.

Perfection of the Patrimony of Wisdom

St. Thomas perfected and augmented still further by the almost angelic quality of his intellect all this superb patrimony of wisdom which he inherited from his predecessors and applied it to prepare, illustrate and protect sacred doctrine in the minds of men (In Librum Boethii de Trinitate, quaest, ii, 3).

Not an Opinion, a Foundation

The reason is that the capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.

Open to Grave Risk

We therefore desired that all teachers of philosophy and sacred theology should be warned that if they deviated so much as a step, in metaphysics especially, from Aquinas, they exposed themselves to grave risk.

Never Shall the Summa Theologica Fall into Disuse

But for the more profound study of this science, as it ought to be studied in Universities and Colleges and in all Seminaries and institutions which are empowered to grant academic degrees, it is of the first importance that the old system of lecturing on the actual text of the Summa Theologica- which should never have been allowed to fall into disuse– be revived.

Present at the Councils

For ever since the happy death of the saintly Doctor, the Church has not held a single Council, but he has been present at it with the wealth of his doctrine.

More in a Year than a Lifetime

“He (Thomas Aquinas) enlightened the Church more than all the other Doctors together; a man can derive more profit from his books in one year than from a lifetime spent in pondering the philosophy of others” (Consistorial address of 1318). - Pope John XXII

Pius X in the Vatican Gardens

Pius X in the Vatican Gardens

The following is the full text of Pope Pius X’s Motu Proprio with the embolden effect and emphases added by SPL.

Doctoris Angelici

Pope Pius X
29 June 1914

Motu Proprio for Italy and the adjacent islands, to encourage the study of the philosophy of St. Thomas Aquinas in Catholic Schools.

No true Catholic has ever ventured to call in question the opinion of the Angelic Doctor that: The regulation of studies is the special concern of the authority of the Holy See by which the universal Church is governed and the need is met by the establishment of Universities (Opusc. Contra impugnantes Dei cultum et religionem, iii). We have already discharged this great duty of Our office elsewhere, and more particularly on the 1st September, 1910, when in the Letter Sacrorum Antistitum, addressed to all Bishops and Superiors of Religious Orders duly charged with the duty of educating young men for the priesthood, We counselled them in the first place as follows: “So far as studies are concerned, it is Our will and We hereby explicitly ordain that the Scholastic philosophy be considered as the basis of sacred studies. . . . And what is of capital importance in prescribing that Scholastic philosophy is to be followed, We have in mind particularly the philosophy which has been transmitted to us by St. Thomas Aquinas. It is Our desire that all the enactments of Our Predecessor in respect thereto be maintained in full force; and, where need be, We renew and confirm them and order them to be strictly observed by all concerned. Let Bishops urge and compel their observance in future in any Seminary in which they may have been neglected. The same injunction applies also to Superiors of Religious Orders.”

Now because the word We used in the text of that letter recommending the philosophy of Aquinas was ‘particularly,’ and not ‘exclusively,’ certain persons persuaded themselves that they were acting in conformity to Our Will or at any rate not actively opposing it, in adopting indiscriminately and adhering to the philosophical opinions of any other Doctor of the School, even though such opinions were contrary to the principles of St. Thomas. They were greatly deceived. In recommending St. Thomas to Our subjects as supreme guide in the Scholastic philosophy, it goes without saying that Our intention was to be understood as referring above all to those principles upon which that philosophy is based as its foundation. For just as the opinion of certain ancients is to be rejected which maintains that it makes no difference to the truth of the Faith what any man thinks about the nature of creation, provided his opinions on the nature of God be sound, because error with regard to the nature of creation begets a false knowledge of God; so the principles of philosophy laid down by St. Thomas Aquinas are to be religiously and inviolably observed, because they are the means of acquiring such a knowledge of creation as is most congruent with the Faith (Contra Gentiles, II, 2, 3); of refuting all the errors of all the ages, and of enabling man to distinguish clearly what things are to be attributed to God and to God alone (ibid., iii; and Sum. Theol., 1, xii, 4: and liv, 1). They also marvellously illustrate the diversity and analogy between God and His works, a diversity and analogy admirably expressed by the Fourth Lateran Council as follows: “The resemblance between the Creator and the creature is such that their still greater dissimilarity cannot fail to be observed” (Decretalis iii, Damnamus ergo, etc. Cf. St. Thomas, Quaest, disp. De Scientia Dei, a. 11). –For the rest, the principles of St. Thomas, considered generally and as a whole, contain nothing but what the most eminent philosophers and doctors of the Church have discovered after prolonged reflection and discussion in regard to the particular reasons determining human knowledge, the nature of God and creation, the moral order and the ultimate end to be pursued in life.

St. Thomas perfected and augmented still further by the almost angelic quality of his intellect all this superb patrimony of wisdom which he inherited from his predecessors and applied it to prepare, illustrate and protect sacred doctrine in the minds of men (In Librum Boethii de Trinitate, quaest, ii, 3). Sound reason suggests that it would be foolish to neglect it and religion will not suffer it to be in any way attenuated. And rightly, because, if Catholic doctrine is once deprived of this strong bulwark, it is useless to seek the slightest assistance for its defence in a philosophy whose principles are either common to the errors of materialism, monism, pantheism, socialism and modernism, or certainly not opposed to such systems. The reason is that the capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.

We therefore desired that all teachers of philosophy and sacred theology should be warned that if they deviated so much as a step, in metaphysics especially, from Aquinas, they exposed themselves to grave risk. –We now go further and solemnly declare that those who in their interpretations misrepresent or affect to despise the principles and major theses of his philosophy are not only not following St. Thomas but are even far astray from the saintly Doctor. If the doctrine of any writer or Saint has ever been approved by Us or Our Predecessors with such singular commendation and in such a way that to the commendation were added an invitation and order to propagate and defend it, it may easily be understood that it was commended to the extent that it agreed with the principles of Aquinas or was in no way opposed to them.

We have deemed it Our apostolic duty to make this declaration and order so that the clergy, both regular and secular, may clearly know Our will and mind in a matter of the gravest importance, and fulfil Our desire with the appropriate alacrity and diligence. Teachers of Christian philosophy and sacred theology will be particularly zealous in this respect, for they must bear in mind that they have not been entrusted with the duty of teaching in order to impart to their pupils whatever opinions they please, but to instruct them in the most approved doctrines of the Church.

As for sacred theology itself, it is Our desire that the study of it be always illuminated by the light of the philosophy before referred to, but in ordinary clerical seminaries, provided suitable teachers are available, there is no objection to the use of text books containing summaries of doctrines derived from the source of Aquinas. There is an ample supply of excellent works of the kind.

But for the more profound study of this science, as it ought to be studied in Universities and Colleges and in all Seminaries and institutions which are empowered to grant academic degrees, it is of the first importance that the old system of lecturing on the actual text of the Summa Theologica- which should never have been allowed to fall into disuse– be revived; for the reason also that prelections on this book make it easier to understand and to illustrate the solemn decrees of the teaching Church and the acts passed in consequence. For ever since the happy death of the saintly Doctor, the Church has not held a single Council, but he has been present at it with the wealth of his doctrine. The experience of so many centuries has shown and every passing day more clearly proves the truth of the statement made by Our Predecessor John XXII: “He (Thomas Aquinas) enlightened the Church more than all the other Doctors together; a man can derive more profit from his books in one year than from a lifetime spent in pondering the philosophy of others” (Consistorial address of 1318). St. Pius V confirmed this opinion when he ordered the feast of St. Thomas as Doctor to be kept by the universal Church: “But inasmuch as, by the providence of Almighty God, the power and truth of the philosophy of the Angelic Doctor, ever since his enrolment amongst the citizens of Heaven, have confounded, refuted and routed many subsequent heresies, as was so often clearly seen in the past and was lately apparent in the sacred decrees of the Council of Trent, We order that the memory of the Doctor by whose valour the world is daily delivered from pestilential errors be cultivated more than ever before with feelings of pious and grateful devotion” (Bull Mirabilis Deus of the 11th April, 1567). To avoid recapitulating the many other resounding praises of Our Predecessors, We may adopt the following words of Benedict XIV as a summary of all the commendations bestowed upon the writings of Thomas Aquinas, more particularly the Summa Theologica: “Numerous Roman Pontiffs, Our Predecessors, have borne glorious testimony to his philosophy. We also, in the books which We have written on various topics, after by diligent examination perceiving and considering the mind of the Angelic Doctor, have always adhered and subscribed with joy and admiration to his philosophy, and candidly confess that whatever good is to be found in Our own Writings is in no way to be attributed to Us, but entirely to so eminent a teacher” (Acta Cap. Gen. O.P., vol IX, p. 196).

A statue of Pope Pius X at St. Peter's Basilica in the Vatican City

Therefore that “the philosophy of St. Thomas may flourish incorrupt and entire in schools, which is very dear to Our heart,” and that “the system of teaching which is based upon the authority and judgement of the individual teacher” and therefore “has a changeable foundation whence many diverse and mutually conflicting opinions arise . . . not without great injury to Christian learning” (Leo XIII, Epist, Qui te of the 19th June, 1886) be abolished forever, it is Our will and We hereby order and command that teachers of sacred theology in Universities, Academies, Colleges, Seminaries and Institutions enjoying by apostolic indult the privilege of granting academic degrees and doctorates in philosophy, use the Summa Theologica of St. Thomas as the text of their prelections and comment upon it in the Latin tongue, and let them take particular care to inspire their pupils with a devotion for it.

Such is already the laudable custom of many Institutions. Such was the rule which the sagacious founders of Religious Orders, with the hearty approval of Our Predecessors, desired should be observed in their own houses of study; and the saintly men who came after the time of St. Thomas Aquinas took him and no other for their supreme teacher of philosophy. So also and not otherwise will theology recover its pristine glory and all sacred studies be restored to their order and value and the province of the intellect and reason flower again in a second spring.

In future, therefore, no power to grant academic degrees in sacred theology will be given to any institution unless Our present prescription is religiously observed therein. Institutions or Faculties of Orders and Regular Congregations, also, already in lawful possession of the power of conferring such academic degrees or similar diplomas, even within the limits of their own four walls, shall be deprived of such a privilege and be considered to have been so deprived if, after the lapse of three years, they shall not have religiously obeyed for any reason whatsoever, even beyond their control, this Our injunction.

This is Our Order, and nothing shall be suffered to gainsay it.

Given at Rome, at St. Peter’s, on the 29th day of June, 1914, the eleventh year of Our Pontificate. Pius PP. X.