Pope Paul VI: “the Christian who wishes to live his faith in a political activity …”
“Therefore the Christian who wishes to live his faith in a political activity … cannot adhere to the Marxist ideology, to its atheistic materialism, to its dialectic of violence and to the way it absorbs individual freedom in the collectivity, at the same time denying all transcendence to man and his personal and collective history; nor can he adhere to the liberal ideology which believes it exalts individual freedom by withdrawing it from every limitation, by stimulating it through exclusive seeking of interest and power, and by considering social solidarities as more or less automatic consequences of individual initiatives, not as an aim and a major criterion of the value of the social organization.”
-Pope Paul VI, in his Apostolic Letter “On the Eightieth Anniversary of the Encyclical ‘Rerum Novarum.'”
You can read the entire Apostolic letter here.