VATICAN CITY (AP) _ Pope John Paul II, in the fifth encyclical of his papacy, today lashed out at atheism and Marxism, saying philosophies that reserve no place for God are really the ''death of man.''

In the 141-page document, the pontiff also condemned what he described as increasing signs of death in the modern world, including poverty and famine, terrorism, the danger of nuclear self-destruction, abortion and euthanasia.

The encyclical, entitled in Latin ''Dominum et Vivificantem'' (The Lord, the Giver of Life), is devoted to the Holy Spirit, the third member of the Holy Trinity, along with the Father and the Son.

An encyclical is the most authoritative form of papal letter, informing the Roman Catholic Church and its members of matters of importance. The Vatican, which provided an official English-language version, said the pope wrote the original in his native Polish.

Vatican spokesman Joaquin Navarro said the document reflects the pontiff's long-held personal views.

''It reflects his idea of sin, not only in social-political terms but on a spiritual level, and how Christians should deal with it as the church charts its course for the third millenium,'' Navarro said.

In the document, John Paul discussed the meaning and the role of the Holy Spirit as Christianity prepares to celebrate the 2,000th anniversary of the birth of Jesus.

The pope said in the introduction that the main purpose of the document is to develop in the church the awareness ''that it is compelled by the Holy Spirit to do its part toward the full realization of the will of God, who has established Christ as the source of salvation for the whole world.''

The encyclical blamed the troubles of the world on ''the evil of disobedience'' to God through ''man's claim to become an independent and exclusive source for deciding about good and evil.''

''We see this confirmed in the modern age, when the atheistic ideologies seek to root out religion on the grounds that religion causes the radical 'alienation' of man,' '' John Paul wrote.

''Hence a process of thought and historic-sociological practice in which the rejection of God has reached the point of declaring his 'death' - an absurdity both in concept and expression,'' the pope said.

But the notion of the ''death of God,'' John Paul said, is more a threat to humanity than anything else.

''The ideology of the 'death of God' easily demonstrates in its effects that ... it is the ideology of the 'death of man,''' the pontiff wrote.

He attacked Marxism for opposing religion ''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.''

The pope said materialism is ''still recognized as the essential core'' of Marxism and is the result of ''the systematic and logical development'' of man's resistance to God.

Therefore, he said, Christians ''cannot but reject'' Marxism as anti- religious materialism. The pope said materialism has placed God ''in a state of suspicion'' and challenges man to become the adversary of God.

That is in line with consistent warnings by the pope and the Vatican against Marxist elements in liberation theology, a movement spearheaded by Catholic priests and nuns who are seek social equality in developing countries.

The pope said the Christians, through prayer and the intercession of the Holy Spirit, should deal with ''the signs and symptoms of death'' he said have become particularly frequent.

Another main theme of the document is Christian unity. The Holy Spirit is described as ''the one who points out ways leading to the union of Christians.''

Noting that the past thousand years have seen the emergence of ''the great separation between Christians,'' the pope urged all who believe in Christ to ''strive to conform their thinking and action to the will of the Holy Spirit.''

He endorsed Pope Paul VI's call in 1973 for a ''new study and devotion to the Holy Spirit,'' and highlights the importance of this study for the ecumenical movement.

Jonh Paul's previous encyclicals were ''Redeemer of Man'' (1979), ''Riches in Mercy'' (1980), ''On Human Work'' (1981) and ''Apostles of the Slavs'' (1985).

The latest document is the third papal encyclical in 100 years on the Holy Spirit. The previous ones were issued by Leo XIII in 1897 and Pius XII in 1943.