Let’s let the Supreme Pontiff answer:

“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.” – Pope Blessed Pius IX Encyclical Nostis et Nobiscum, December 8, 1849

“…communism, socialism, nihilism, hideous deformities of the civil society of men and almost its ruin.” – Pope Leo XIII Encyclical Diuturnum, June 29, 1881

“…socialists and members of other seditious societies, who labor unceasingly to destroy the State even to its foundations.” – Pope Leo XIII Libertas Praestantissimum, June 20, 1888

“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 25, 1910, condemning the movement Le Sillon)

“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.” – Benedict XV Encyclical Ad Beatissimi Apostolorum, November 1, 1914, n. 13

“… 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.” –Pope Pius XI Quadragesimo Anno, May 15, 1931, n. 111

“[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.” –Pope Pius XI Quadragesimo Anno, May 15, 1931, n. 120

“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.” – Pope Pius XII Summi Pontificatus, October 20, 1939, n. 6

“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.” – Pope Saint John XXIII Mater et Magistra, May 15, 1961, n. 34)

“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.” – Pope Paul VI Apostolic Letter Octogesima Adveniens, May 14, 1971, n. 31)

“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.’” – Pope Saint John Paul II Centesimus Annus − On the 100th anniversary of Pope Leo XIII’s Rerum Novarum, May 1, 1991, n. 12

“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.” – Pope Benedict XVI Encyclical Deus Caritas Est, December 25, 2005, n. 28

“Too few have been able to grasp the nature of Communism. The majority instead succumb to its deception, skillfully concealed by the most extravagant promises. By pretending to desire only the betterment of the condition of the working classes, by urging the removal of the very real abuses chargeable to the liberalistic economic order, and by demanding a more equitable distribution of this world’s goods (objectives entirely and undoubtedly legitimate), the Communist takes advantage of the present world-wide economic crisis to draw into the sphere of his influence, even those sections of the populace which on principle reject all forms of materialism and terrorism.” – Pope Pius XI, “Divini Redemptoris” (1937)

“…Communism, as it is called, [is] a doctrine most opposed to the very natural law. For if this doctrine were accepted, the complete destruction of everyone’s laws, government, property, and even of human society itself would follow.” – Pope Blessed Pius IX, “Qui Pluribus” (1846)

Then of course, there was the Papal Decree Against Communism published in 1949 by the office of Pope Pius XII:

Q.1 Whether it is lawful to give a name to communist parties or to favor them.
        [By chance is it licit to give name or to make favors to communist parties?]

R. Negative: For communism is materialistic and anti-Christian; and the leaders of the communists, although they sometimes profess in words that they do not attack religion, yet, whether by doctrine or action, they show that they are seriously offended against God and true religion and the Church of Christ.

Q.2 Whether it is lawful to publish, propagate or read books, periodicals, diaries or newspapers, such as the doctrine or activity of the Communists, or to write in them.
       [By chance is it lawful to publish, promulgate or read books, journals or leaflets which defend the action or the communist doctrine, or to write for them?]

R. Negative: For they are forbidden by the very law.
        [Can Christians who perform the acts mentioned on n.1 and 2 be admitted to the sacraments?]

R. Negative, according to the ordinary principles of denying those sacraments, those who are not disposed to

Christi fideles, Who the materialistic and anti-Christian doctrine of the communists they profess, and in the first place, those who defend or propagate it, automatically, as apostates from the Catholic faith, incur the excommunication reserved in a special way to the Apostolic See.
       [If Christians declare openly the materialist and anti-Christian doctrine of the communists, and, mainly, if they defend it or promulgate it, “ipso facto”, do they incur in excommunication (“speciali modo”) reserved to the Apostolic See?]

R. Affirmative