"Atheism in legislation, indifference in matters of religion, and the pernicious maxims which go under the name of Liberal Catholicism are the true causes of the destruction of states; they have been the ruin of France. Believe me, the evil I denounce is more terrible than the Revolution, more terrible even than The Commune. I have always condemned Liberal Catholicism, and I will condemn it again forty times over if it be necessary."
In a brief, 6th of March, 1873, addressed to the Circle of St. Ambrose of Milan, the Sovereign Pontiff thus expresses himself:
"People are not wanting who pretend to form an alliance between light and darkness and to associate justice with iniquity in favor of those doctrines called Liberal Catholicism, which, based on the most pernicious principles, show themselves favorable to the intrusion of secular power upon the domain of spirituals; they lead their partisans to esteem, or at least to tolerate, iniquitous laws, as if it were not written that no one can serve two masters. Those who thus conduct themselves are more dangerous and more baneful than declared enemies, not only because, without being warned of it, perhaps even without being conscious of it, they second the projects of wicked men, but also because, keeping within certain limits, they show themselves with some appearance of probity and sound doctrine. They thus deceive the indiscreet friends of conciliation and seduce honest people, who would otherwise have strenuously combated a declared error."
In the Brief of the 8th of May of the same year, speaking to the Confederation of the Catholic Circle of Belgium, the same Holy Father said:
"What we praise above all in your religious enterprise is the absolute aversion which, as we are informed, you show towards the principles of Liberal Catholicism and your intrepid determination to root them out as soon as possible. In truth you will extirpate the fatal root of discord and you will efficaciously contribute to unite and strengthen the minds of all in so combating this insidious error, much more dangerous than an open enemy because it hides itself under the specious veil of zeal and of charity, and is so endeavoring to protect the people in general from its contaminating influence. Surely you, who adhere with such complete submission to all decisions of this Apostolic Seat and who know its frequent reprobations of Liberal principles, have no need of these warnings."
In the Brief to the La Croix, a Belgium journal, on the 24th of May, 1874, the Pope expresses himself thus:
"We cannot do less than to praise the design expressed in this letter, which we know your journal will satisfactorily fulfill, the design to publish, to spread, to comment on and inculcate in all minds all that the Holy See teaches against the perverse or at least false doctrines professed in so many quarters, and particularly against Liberal Catholicism, bitterly striving to conciliate light with darkness and truth with error."
Source: What is Liberalism, Imprimatur 1899