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You may also wish to look at an encyclical letter published by his successor, Pio X: Pascendi Dominici Gregis.

In that admirable work, he stated, among other things:

Still it must be confessed that the number of the enemies of the cross of Christ has in these last days increased exceedingly, who are striving, by arts, entirely new and full of subtlety, to destroy the vital energy of the Church, and, if they can, to overthrow utterly Christ's kingdom itself. Wherefore We may no longer be silent, lest We should seem to fail in Our most sacred duty, and lest the kindness that, in the hope of wiser counsels, We have hitherto shown them, should be attributed to forgetfulness of Our office.
Gravity of the Situation
2. That We make no delay in this matter is rendered necessary especially by the fact that the partisans of error are to be sought not only among the Church's open enemies; they lie hid, a thing to be deeply deplored and feared, in her very bosom and heart, and are the more mischievous, the less conspicuously they appear. We allude, Venerable Brethren, to many who belong to the Catholic laity, nay, and this is far more lamentable, to the ranks of the priesthood itself, who, feigning a love for the Church, lacking the firm protection of philosophy and theology, nay more, thoroughly imbued with the poisonous doctrines taught by the enemies of the Church, and lost to all sense of modesty, vaunt themselves as reformers of the Church; and, forming more boldly into line of attack, assail all that is most sacred in the work of Christ, not sparing even the person of the Divine Redeemer, whom, with sacrilegious daring, they reduce to a simple, mere man.
3. Though they express astonishment themselves, no one can justly be surprised that We number such men among the enemies of the Church, if, leaving out of consideration the internal disposition of soul, of which God alone is the judge, he is acquainted with their tenets, their manner of speech, their conduct. Nor indeed will he err in accounting them the most pernicious of all the adversaries of the Church. For as We have said, they put their designs for her ruin into operation not from without but from within; hence, the danger is present almost in the very veins and heart of the Church, whose injury is the more certain, the more intimate is their knowledge of her. Moreover they lay the axe not to the branches and shoots, but to the very root, that is, to the faith and its deepest fires. And having struck at this root of immortality, they proceed to disseminate poison through the whole tree, so that there is no part of Catholic truth from which they hold their hand, none that they do not strive to corrupt. Further, none is more skilful, none more astute than they, in the employment of a thousand noxious arts; for they double the parts of rationalist and Catholic, and this so craftily that they easily lead the unwary into error; and since audacity is their chief characteristic, there is no conclusion of any kind from which they shrink or which they do not thrust forward with pertinacity and assurance. To this must be added the fact, which indeed is well calculated to deceive souls, that they lead a life of the greatest activity, of assiduous and ardent application to every branch of learning, and that they possess, as a rule, a reputation for the strictest morality. Finally, and this almost destroys all hope of cure, their very doctrines have given such a bent to their minds, that they disdain all authority and brook no restraint; and relying upon a false conscience, they attempt to ascribe to a love of truth that which is in reality the result of pride and obstinacy.
Once indeed We had hopes of recalling them to a better sense, and to this end we first of all showed them kindness as Our children, then we treated them with severity, and at last We have had recourse, though with great reluctance, to public reproof. But you know, Venerable Brethren, how fruitless has been Our action. They bowed their head for a moment, but it was soon uplifted more arrogantly than ever. If it were a matter which concerned them alone, We might perhaps have overlooked it: but the security of the Catholic name is at stake. Wherefore, as to maintain it longer would be a crime, We must now break silence, in order to expose before the whole Church in their true colours those men who have assumed this bad disguise.

Both Leo XIII and Pio X recognized the influence of liberalism in the destruction of society. For example, in Libertas, Leo XIII stated,

15. What naturalists or rationalists aim at in philosophy, that the supporters of liberalism, carrying out the principles laid down by naturalism, are attempting in the domain of morality and politics. The fundamental doctrine of rationalism is the supremacy of the human reason, which, refusing due submission to the divine and eternal reason, proclaims its own independence, and constitutes itself the supreme principle and source and judge of truth. Hence, these followers of liberalism deny the existence of any divine authority to which obedience is due, and proclaim that every man is the law to himself; from which arises that ethical system which they style independent morality, and which, under the guise of liberty, exonerates man from any obedience to the commands of God, and substitutes a boundless license. The end of all this it is not difficult to foresee, especially when society is in question. For, when once man is firmly persuaded that he is subject to no one, it follows that the efficient cause of the unity of civil society is not to be sought in any principle external to man, or superior to him, but simply in the free will of individuals; that the authority in the State comes from the people only; and that, just as every man's individual reason is his only rule of life, so the collective reason of the community should be the supreme guide in the management of all public affairs. Hence the doctrine of the supremacy of the greater number, and that all right and all duty reside in the majority. But, from what has been said, it is clear that all this is in contradiction to reason. To refuse any bond of union between man and civil society, on the one hand, and God the Creator and consequently the supreme Law-giver, on the other, is plainly repugnant to the nature, not only of man, but of all created things; for, of necessity, all effects must in some proper way be connected with their cause; and it belongs to the perfection of every nature to contain itself within that sphere and grade which the order of nature has assigned to it, namely, that the lower should be subject and obedient to the higher.

Pio X said in Pascendi Dominici Gregis,

10. Therefore the religious sentiment, which through the agency of vital immanence emerges from the lurking places of the subconsciousness, is the germ of all religion, and the explanation of everything that has been or ever will be in any religion. The sentiment, which was at first only rudimentary and almost formless, gradually matured, under the influence of that mysterious principle from which it originated, with the progress of human life, of which, as has been said, it is a form. This, then, is the origin of all religion, even supernatural religion; it is only a development of this religious sentiment. Nor is the Catholic religion an exception; it is quite on a level with the rest; for it was engendered, by the process of vital immanence, in the consciousness of Christ, who was a man of the choicest nature, whose like has never been, nor will be. - Those who hear these audacious, these sacrilegious assertions, are simply shocked! And yet, Venerable Brethren, these are not merely the foolish babblings of infidels. There are many Catholics, yea, and priests too, who say these things openly; and they boast that they are going to reform the Church by these ravings! There is no question now of the old error, by which a sort of right to the supernatural order was claimed for the human nature. We have gone far beyond that: we have reached the point when it is affirmed that our most holy religion, in the man Christ as in us, emanated from nature spontaneously and entirely. Than this there is surely nothing more destructive of the whole supernatural order. Wherefore the Vatican Council most justly decreed: "If anyone says that man cannot be raised by God to a knowledge and perfection which surpasses nature, but that he can and should, by his own efforts and by a constant development, attain finally to the possession of all truth and good, let him be anathema" (De Revel., can. 3).

Both of these two seminal documents attack the same issue: liberalism. Leo XIII looks in the more general direction, while Pio X aims like a laser beam at its destructive influence within the Church.

Small wonder that neither of these documents are rarely, if ever, discussed from the Ambo. You should check (unless you are in a very traditional parish) to see if your clergy, both priests and deacons, have read either of these documents. I would wager that few, if any, have.