Pope Benedict, in an encyclical released on Friday, said atheism was responsible for some of the "greatest forms of cruelty and violations of justice" in history.
The 75-page "Spe Salvi", which takes its Latin title from a quote by St Paul (in hope we were saved), is an appeal to a pessimistic world to find strength in Christian hope.
In the second encyclical of his papacy, Benedict urges Christians to put their hope for the future in God and not in technology, wealth or political ideologies.
Atheism could be regarded by some as a "type of moralism", particularly in the 19th and 20th centuries, to protest against the injustices of the world and world history, he said.
Reciting arguments made by atheists, he said: "A world marked by so much injustice, innocent suffering and cynicism of power cannot be the work of a good God. A God with responsibility for such a world would not be a just God, much less a good God."
History has proven wrong ideologies such as Marxism which say humans had to establish social justice because God did not exist, the Pope wrote.
"It is no accident that this idea has led to the greatest forms of cruelty and violations of justice," the Pope said. Such a concept was grounded in "intrinsic falsity".
Marxism, the Pope wrote, had left behind "a trail of appalling destruction" because it failed to realise that man could not be "merely the product of economic conditions".
ATHEISTS REACT
The encyclical is the highest form of papal writing and addresses all members of the Church. This document is written in a highly academic, professorial style in which the Pope quotes saints, philosophers and writers to make his point.
Atheism has been a hot topic recently thanks to best-selling books questioning the value of religion such as "The God Delusion" by Richard Dawkins and "God is Not Great" by Christopher Hitchens.
The Pope seemed to be addressing the fresh interest in atheism in the developed world with phrases such as: "Let us put it very simply: man needs God, otherwise he remains without hope."
Italy's Union of Atheists, Agnostics and Rationalists (UAAR) said by taking such stands the Pope would push more people away from the Church.
"The existence of a billion non-believers in the world should be enough to make the Pope understand that man can live very well without God, but with reason," a statement said.
The Pope urged Christians to put their hope for a better future in God.
"We have all witnessed the way in which progress, in the wrong hands, can become and has indeed become a terrifying progress in evil. If technical progress is not matched by corresponding progress in man's ethical formation, in man's inner growth, then it is not progress at all, but a threat for man and for the world," he said.
Christian hope also meant protecting the planet, even if people felt powerless to make changes in their lifetimes, he said.
"We can free our life and the world from the poisons and contaminations that could destroy the present and the future. We can uncover the sources of creation and keep them unsullied, and in this way we can make a right use of creation, which comes to us as a gift..." he said.
