Pope Leo XIII, in his 1878 encyclical "Quod Apostolici Muneris," condemned socialism and communism as dangerous ideologies that threatened social order and individual freedom. He saw these ideologies as detrimental to the natural order of society, arguing they would lead to widespread confusion and the overthrow of established institutions. 

Key Points of Leo XIII's Condemnation:

Threat to Social Order:
Leo XIII believed that socialism and communism aimed to dismantle the existing social structure and create a new order based on collectivism and class struggle. 

Erosion of Individual Rights:
He argued that these ideologies would strip individuals of their natural rights, including the right to property, and reduce them to mere cogs in a machine. 

Attack on Family and Religion:
Leo XIII warned that socialism and communism would undermine the sanctity of marriage and family, replacing them with a purely civil and materialistic view. 

Distortion of Justice:
He criticized the socialist and communist proposals for redistributing wealth and property, arguing that they would rob lawful possessors of their rights and distort the functions of the state. 

Natural Harmony:
Leo XIII emphasized that nature ordained that the rich and the working class should live in harmony and agreement, not in conflict. 

Need for Just Social Order:
He called for a just social order that would protect the rights of both the worker and the capitalist, promoting a regulated capitalism that ensured fair wages and decent working conditions. 

https://www.papalencyclicals.net/pius09/p9nostis.htm
https://www.vatican.va/content/leo-xiii/en/encyclicals/documents/hf_l-xiii_enc_15051891_rerum-novarum.html
https://www.marxists.org/archive/marx/works/download/pdf/Manifesto.pdf
https://psu.pb.unizin.org/lessonslearned/chapter/a-lesson-from-pope-leo-xiii/

"It’s no wonder Pope Leo XIII denounced those who “think that the inherent character of marriage can be perverted with impunity; and who, disregarding the sanctity of religion and of the sacrament.” He warned that both private families and public society risked being “miserably driven into that general confusion and overthrow of order which is even now the wicked aim of socialists and communists” (Arcanum, 32)."
https://www.catholic.com/magazine/print-edition/why-socialism-is-immoral

"3. This all too imminent danger, Venerable Brethren, as you have already surmised, is bolshevistic and atheistic Communism, which aims at upsetting the social order and at undermining the very foundations of Christian civilization."
https://www.vatican.va/content/pius-xi/en/encyclicals/documents/hf_p-xi_enc_19370319_divini-redemptoris.html

"Using this remedy to resolve poor conditions for the laborer, the pontiff contends, “is manifestly against justice” because “every man has by nature the right to possess property as his own.” He further argues that government intrusion into the rights of property and the right to provide for one’s family is “a great and pernicious error.”"
https://archsa.org/pope-leo-xiiis-encyclical-promoting-labor-and-condemning-socialism-rerum-novarum/

"Any truly solicitous regime, Salazar insisted, must turn its energies towards understanding the true, objective foundations of social order. Only when the real order of things was enshrined in law could justice and freedom be assured, and injury to man’s rational faculties be avoided." 
https://media.christendom.edu/1985/07/portugal-austria-and-catholic-counter-revolution-in-interwar-europe/#:~:text=Any%20truly%20solicitous%20regime%2C%20Salazar%20insisted%2C%20must,injury%20to%20man%27s%20rational%20faculties%20be%20avoided.