Opposition to God, who is an invisible Spirit, to a certain degree originates in the very fact of the radical difference of the world from God, that is to say in the world's "visibility" and "materiality" in contrast to him who is "invisible" and "absolute Spirit"; from the world's essential and inevitable imperfection in contrast to him, the perfect being. But this opposition becomes conflict and rebellion on the ethical plane by reason of that sin which takes possession of the human heart, wherein "the desires of the flesh are against the Spirit and the desires of the Spirit are against the flesh." Concerning this sin, the Holy Spirit must "convince the world," as we have already said. 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. (Papal Encyclical, Dominum et Vivificantem, 18 May 1986).