“Overthrow [of] the entire order of human affairs”
“You are aware indeed, that the goal of this most iniquitous plot is to drive people to overthrow the entire order of human affairs and to draw them over to the wicked theories of this Socialism and Communism, by confusing them with perverted teachings.” (Encyclical Nostis et Nobiscum, December 8, 1849)
 

Hideous monster
“...communism, socialism, nihilism, hideous deformities of the civil society of men and almost its ruin.” (Encyclical Diuturnum, June 29, 1881)

Ruin of all institutions
“... For, the fear of God and reverence for divine laws being taken away, the authority of rulers despised, sedition permitted and approved, and the popular passions urged on to lawlessness, with no restraint save that of punishment, a change and overthrow of all things will necessarily follow. Yea, this change and overthrow is deliberately planned and put forward by many associations of communists and socialists” (Encyclical Humanum Genus, April 20, 1884, n. 27).

A sect “that threatens civil society with destruction”

The dream of re-shaping society will bring socialism
“But stranger still, alarming and saddening at the same time, are the audacity and frivolity of men who call themselves Catholics and dream of re-shaping society under such conditions, and of establishing on earth, over and beyond the pale of the Catholic Church, ‘the reign of love and justice’ ... What are they going to produce? ... A mere verbal and chimerical construction in which we shall see, glowing in a jumble, and in seductive confusion, the words Liberty, Justice, Fraternity, Love, Equality, and human exultation, all resting upon an ill-understood human dignity. It will be a tumultuous agitation, sterile for the end proposed, but which will benefit the less Utopian exploiters of the people. Yes, we can truly say that the Sillon, its eyes fixed on a chimera, brings Socialism in its train.” (Apostolic Letter Notre Charge Apostolique ["Our Apostolic Mandate"] to the French Bishops, August 15, 1910, condemning the movement Le Sillon)

 

The condemnation of socialism should never be forgotten
“It is not our intention here to repeat the arguments which clearly expose the errors of Socialism and of similar doctrines. Our predecessor, Leo XIII, most wisely did so in truly memorable Encyclicals; and you, Venerable Brethren, will take the greatest care that those grave precepts are never forgotten, but that whenever circumstances call for it, they should be clearly expounded and inculcated in Catholic associations and congresses, in sermons and in the Catholic press.” (Encyclical Ad Beatissimi Apostolorum, November 1, 1914, n. 13)

Socialism, fundamentally contrary to Christian truth
“... For Socialism, which could then be termed almost a single system and which maintained definite teachings reduced into one body of doctrine, has since then split chiefly into two sections, often opposing each other and even bitterly hostile, without either one however abandoning a position fundamentally contrary to Christian truth that was characteristic of Socialism.” (Encyclical Quadragesimo Anno, May 15, 1931, n. 111)
 

Socialism cannot be reconciled with Catholic Doctrine
“But what if Socialism has really been so tempered and modified as to the class struggle and private ownership that there is in it no longer anything to be censured on these points? Has it thereby renounced its contradictory nature to the Christian religion? This is the question that holds many minds in suspense. And numerous are the Catholics who, although they clearly understand that Christian principles can never be abandoned or diminished seem to turn their eyes to the Holy See and earnestly beseech Us to decide whether this form of Socialism has so far recovered from false doctrines that it can be accepted without the sacrifice of any Christian principle and in a certain sense be baptized. That We, in keeping with Our fatherly solicitude, may answer their petitions, We make this pronouncement: Whether considered as a doctrine, or an historical fact, or a movement, Socialism, if it remains truly Socialism, even after it has yielded to truth and justice on the points which we have mentioned, cannot be reconciled with the teachings of the Catholic Church because its concept of society itself is utterly foreign to Christian truth.” (Ibid. n. 117)

Catholic Socialism, a contradiction
“[Socialism] is based nevertheless on a theory of human society peculiar to itself and irreconcilable with true Christianity. Religious socialism, Christian socialism, are contradictory terms; no one can be at the same time a good Catholic and a true socialist.” (Ibid. n. 120)
 

The Church will fight to the end, in defense of supreme values threatened by socialism
“[The Church undertook] the protection of the individual and the family against a current threatening to bring about a total socialization which in the end would make the specter of the 'Leviathan' become a shocking reality. The Church will fight this battle to the end, for it is a question of supreme values: the dignity of man and the salvation of souls." (“Radio message to the Katholikentag of Vienna,” September 14, 1952 in Discorsi e Radiomessaggi, vol. XIV, p. 314)

The state can not be regarded as being above all
"To consider the State as something ultimate to which everything else should be subordinated and directed, cannot fail to harm the true and lasting prosperity of nations." (Encyclical Summi Pontificatus, October 20, 1939, n. 60)
 

“No Catholic could subscribe even to moderate socialism”
“Pope Pius XI further emphasized the fundamental opposition between Communism and Christianity, and made it clear that no Catholic could subscribe even to moderate Socialism. The reason is that Socialism is founded on a doctrine of human society which is bounded by time and takes no account of any objective other than that of material well-being. Since, therefore, it proposes a form of social organization which aims solely at production, it places too severe a restraint on human liberty, at the same time flouting the true notion of social authority.” (Encyclical Mater et Magistra, May 15, 1961, n. 34) 

Paul VI

Too often Christians tend to idealize socialism
“Too often Christians attracted by socialism tend to idealize it in terms which, apart from anything else, are very general: a will for justice, solidarity and equality. They refuse to recognize the limitations of the historical socialist movements, which remain conditioned by the ideologies from which they originated.” (Apostolic Letter Octogesima Adveniens, May 14, 1971, n. 31)
 

Socialism: Danger of a “simple and radical solution”
“It may seem surprising that ‘socialism’ appeared at the beginning of the Pope's critique of solutions to the ‘question of the working class’ at a time when ‘socialism’ was not yet in the form of a strong and powerful State, with all the resources which that implies, as was later to happen. However, he correctly judged the danger posed to the masses by the attractive presentation of this simple and radical solution to the ‘question of the working class.’" (Encyclical Centesimus Annus - On the 100^th anniversary of Pope Leo XIII's Rerum Novarum, May 1, 1991, n. 12)

Fundamental error of socialism: A mistaken conception of the person
“Continuing our reflections, ... we have to add that the fundamental error of socialism is anthropological in nature. Socialism considers the individual person simply as an element, a molecule within the social organism, so that the good of the individual is completely subordinated to the functioning of the socio-economic mechanism. Socialism likewise maintains that the good of the individual can be realized without reference to his free choice, to the unique and exclusive responsibility which he exercises in the face of good or evil. Man is thus reduced to a series of social relationships, and the concept of the person as the autonomous subject of moral decision disappears, the very subject whose decisions build the social order. From this mistaken conception of the person there arise both a distortion of law, which defines the sphere of the exercise of freedom, and an opposition to private property.” (Ibid, n. 13)
 

“We do not need a State which regulates and controls everything”

Benedict XVI

“The State which would provide everything, absorbing everything into itself, would ultimately become a mere bureaucracy incapable of guaranteeing the very thing which the suffering person - every person - needs: namely, loving personal concern. We do not need a State which regulates and controls everything, but a State which, in accordance with the principle of subsidiarity, generously acknowledges and supports initiatives arising from the different social forces and combines spontaneity with closeness to those in need. … In the end, the claim that just social structures would make works of charity superfluous masks a materialist conception of man: the mistaken notion that man can live ‘by bread alone’ (Mt 4:4; cf. Dt 8:3) - a conviction that demeans man and ultimately disregards all that is specifically human.” (Encyclical Deus Caritas Est, December 25, 2005, n. 28)