St. John Paul II ... Holy Spirit Encyclical - Part III -Section 3, paragraph 56
56. Unfortunately, the resistance to the Holy Spirit which St. Paul emphasizes in the interior and subjective dimension as tension, struggle and rebellion taking place in the human heart, finds in every period of history and especially in the modern era its external dimension, which takes concrete form as the content of culture and civilization, as a philosophical system, an ideology, a program for action and for the shaping of human behavior. It reaches its clearest expression in materialism, both in its theoretical form: as a system of thought, and in its practical form: as a method of interpreting and evaluating facts, and likewise as a program of corresponding conduct. The system which has developed most and carried to its extreme practical consequences this form of thought, ideology and praxis is dialectical and historical materialism, which is still recognized as the essential core of Marxism.
In principle and in fact, materialism radically excludes the presence and action of God, who is spirit, in the world and above all in man. Fundamentally this is because it does not accept God's existence, being a system that is essentially and systematically atheistic. This is the striking phenomenon of our time: atheism, to which the Second Vatican Council devoted some significant pages.242 Even though it is not possible to speak of atheism in a univocal way or to limit it exclusively to the philosophy of materialism, since there exist numerous forms of atheism and the word is perhaps often used in a wrong sense, nevertheless it is certain that a true and proper materialism, understood as a theory which explains reality and accepted as the key-principle of personal and social action, is characteristically atheistic. The order of values and the aims of action which it describes are strictly bound to a reading of the whole of reality as "matter." Though it sometimes also speaks of the "spirit" and of "questions of the spirit," as for example in the fields of culture or morality, it does so only insofar as it considers certain facts as derived from matter (epiphenomena), since according to this system matter is the one and only form of being. It follows, according to this interpretation, that religion can only be understood as a kind of "idealistic illusion," to be fought with the most suitable means and methods according to circumstances of time and place, in order to eliminate it from society and from man's very heart.
It can be said therefore that materialism is the systematic and logical development of that resistance" and opposition condemned by St. Paul with the words: "The desires of the flesh are against the Spirit." But, as St. Paul emphasizes in the second part of his aphorism, this antagonism is mutual: "The desires of the Spirit are against the flesh." Those who wish to live by the Spirit, accepting and corresponding to his salvific activity, cannot but reject the internal and external tendencies and claims of the "flesh," also in its ideological and historical expression as anti-religious "materialism." Against this background so characteristic of our time, in preparing for the great Jubilee we must emphasize the "desires of the spirit," as exhortations echoing in the night of a new time of advent. at the end of which, like two thousand years ago, "every man will see the salvation of God."243 This is a possibility and a hope that the Church entrusts to the men and women of today. She knows that the meeting or collision between the "desires against the spirit" which mark so many aspects of contemporary civilization, especially in some of its spheres, and "the desires against the flesh," with God's approach to us, his Incarnation, his constantly renewed communication of the Holy Spirit-this meeting or collision may in many cases be of a tragic nature and may perhaps lead to fresh defeats for humanity. But the Church firmly believes that on God's part there is always a salvific self-giving, a salvific coming and, in some way or other, a salvific "convincing concerning sin" by the power of the Spirit.