“See to it, Venerable Brethren,” Pope Pius XI warned in his 1937 encyclical Divini Redemptoris, “that the Faithful do not allow themselves to be deceived! Communism is intrinsically wrong, and no one who would save Christian civilization may collaborate with it in any undertaking whatsoever. Those who permit themselves to be deceived into lending their aid towards the triumph of Communism in their own country, will be the first to fall victims of their error.”
Pius XI was simply reiterating what the Church had taught about communism since the 19th century: it was full of errors, contrary to the good of the social order, and incompatible with the Christian religion.
Here is a non-exhaustive list of 5 reasons why the Church opposes communism:
1) Private property is a natural human right
Private property is not a social construct that can be done away with at will. Rather, it is a part of the natural social order and is necessary for society to be healthy and just.
Private property is the natural fruit of work and protects human dignity since it “assures a person a highly necessary sphere for the exercise of his personal and family autonomy.” Thus, it is “an essential element of an authentically social and democratic economic policy, and it is the guarantee of a correct social order.” (CSDC 176)
Programs to eliminate private property, Pope Leo XIII warned in his 1891 encyclical Rerum Novarum, are “emphatically unjust, for they would rob the lawful possessor, distort the functions of the State, and create utter confusion in the community.”
2) Communism violates subsidiarity
Communism involves the centralized control of key aspects of society. This violates subsidiarity, one of the most important principles of Catholic social thought.
What is subsidiarity? It is, according to Pius XI, “that most weighty principle, which cannot be set aside or changed, [and] remains fixed and unshaken in social philosophy,” and it is defined thusly: “Just as it is gravely wrong to take from individuals what they can accomplish by their own initiative and industry and give it to the community, so also it is an injustice and at the same time a grave evil and disturbance of right order to assign to a greater and higher association what lesser and subordinate organizations can do. For every social activity ought of its very nature to furnish help to the members of the body social, and never destroy and absorb them.” (Quadragesimo Anno, 79)
3) Persecution of the Church
Because the Church rejects communism, rejects earthly utopian visions, believes in God, and represents a power center apart from the government, communist countries have always attacked and tried to suppress the Catholic Church. The Soviet Union alone is estimated to have killed millions of Christians.
Since the the Church is the divinely established organization for the salvation of the world, this is of course gravely evil.
Communists usually explicitly deny the existence of God and seek to build society around this denial. Atheism is of course false and any worldview built on it is doomed to failure, injustice, and destruction.
5) Class warfare is wrong and not inevitable
Class warfare encourages the deadly sin of envy and is based on a destructive us-versus-them mentality. Rather, all people, regardless of their state, are called to cooperation in charity.
“The great mistake… [is] the notion that class is naturally hostile to class,” Leo XIII teaches in Rerum Novarum, “and that the wealthy and the working men are intended by nature to live in mutual conflict. So irrational and so false is this view that the direct contrary is the truth. […] Each needs the other: capital cannot do without labor, nor labor without capital. Mutual agreement results in the beauty of good order, while perpetual conflict necessarily produces confusion and savage barbarity.” (19)