“Among the Scholastic Doctors, the chief and master of all towers Thomas Aquinas, who, as Cajetan observes, because "he most venerated the ancient Doctors of the Church, in a certain way seems to have inherited the intellect of all."
The doctrines of those illustrious men, like the scattered members of a body, Thomas collected together and cemented, distributed in wonderful order, and so increased with important additions that he is rightly and deservedly esteemed the special bulwark and glory of the Catholic faith.
With his spirit at once humble and swift, his memory ready and tenacious, his life spotless throughout, a lover of truth for its own sake, richly endowed with human and divine science, like the sun he heated the world with the warmth of his virtues and filled it with the splendor of his teaching.
Philosophy has no part which he did not touch finely at once and thoroughly; on the laws of reasoning, on God and incorporeal substances, on man and other sensible things, on human actions and their principles, he reasoned in such a manner that in him there is wanting neither a full array of questions, nor an apt disposal of the various parts, nor the best method of proceeding, nor soundness of principles or strength of argument, nor clearness and elegance of style, nor a facility for explaining what is abstruse.
Moreover, the Angelic Doctor pushed his philosophic inquiry into the reasons and principles of things, which because they are most comprehensive and contain in their bosom, so to say, the seeds of almost infinite truths, were to be unfolded in good time by later masters and with a goodly yield.
And as he also used this philosophic method in the refutation of error, he won this title to distinction for himself: that, single-handed, he victoriously combated the errors of former times, and supplied invincible arms to put those to rout which might in after-times spring up.
Again, clearly distinguishing, as is fitting, reason from faith, while happily associating the one with the other, he both preserved the rights and had regard for the dignity of each; so much so, indeed, that reason, borne on the wings of Thomas to its human height, can scarcely rise higher, while faith could scarcely expect more or stronger aids from reason than those which she has already obtained through Thomas.
For these reasons most learned men, in former ages especially, of the highest repute in theology and philosophy, after mastering with infinite pains the immortal works of Thomas, gave themselves up not so much to be instructed in his angelic wisdom as to be nourished upon it.
It is known that nearly all the founders and lawgivers of the religious orders commanded their members to study and religiously adhere to the teachings of St. Thomas, fearful least any of them should swerve even in the slightest degree from the footsteps of so great a man. To say nothing of the family of St. Dominic, which rightly claims this great teacher for its own glory, the statutes of the Benedictines, the Carmelites, the Augustinians, the Society of Jesus, and many others all testify that they are bound by this law. "
- From Pope Leo XIII - Aeterni Patris (August 4, 1879)