Pope Benedict XVI blasted atheism in a new document released on Friday, saying that in modern times it had produced terrible cruelty and injustice.
In his second encyclical letter, the pope said that the ideologies and revolutions of the 19th and 20th centuries had failed to satisfy humanity's search for justice and that Christian hope was the answer.
The 77-page document, entitled Spe Salvi (In hope we were saved) and published on Friday, is a complex academic text, arranged in 50 paragraphs and littered with quotations.
It explores issues such as the relationships between hope and faith; dialogue between faith and reason; and the way in which the concepts of progress, science and freedom intertwine.
The pope argues that the key ideologies of the last two centuries have been based on the hope of establishing justice in an unjust world without reference to God.
Benedict said this was perhaps understandable but any attempt was destined to failure because removing God meant killing hope: ''A world that makes its own justice is a world without hope''.
''If in the face of the suffering of this world a protest against God is comprehensible, the claim that humanity can and should do what no God does, or is able to do, is presumptious and intrinsically false''.
Hopes of a just but godless order of things produced atheistic ideologies which had caused ''the greatest forms of cruelty and violations of justice,'' said the pontiff, who grew up in Nazi Germany.
Benedict highlighted Marxism in particular, accusing it of leaving behind it a ''trail of appalling destruction''.
''Marx's fundamental error was to forget man and his freedom. He thought that once the economy had been put right, everything would automatically be put right. His real error is materialism''.
EXCESSIVE DEVOTION TO REASON.
Materialism was an excessive devotion to reason and rationality, the pope said. It offered an incomplete view of existence and inadequate solutions to the problems facing humanity.
This was why progress was ''ambiguous'' if it was only technological or scientific, the pope said, stressing that until ethics came into play the risks were enormous.
''It (progress) offers new possibilities for good but opens up appalling possibilities for evil, possibilities that formerly did not exist. We know that in the wrong hands progress can become and has become a terrible progress in evil''.
These remarks appeared to refer partly to deep Catholic objections to research in many fields of medical science, including stem cells and genetic engineering.
Spe Salvi comes in the wake of a clutch of best-selling books which have set out to show that God does not exist and to criticise religious belief.
They include The God Delusion by Richard Dawkins, Sam Harris's The End of Faith, Daniel Dennett's Breaking the Spell and God Is Not Great by Christopher Hitchens.
One of the first reactions to the pope's new encyclical came from Italy's Union of Atheists and Rationalist Agnostics.
''The pope can relax: atheists don't feel they have no hope. Far from it. They are fine as they are, living only within the limits of their life on earth, without having to reassure themselves with the hope of a blissful afterlife,'' said secretary Raffaele Carcano.
Bristling over the pope's references to the ''cruelty'' and ''violations of justice'' resulting from atheism, he urged Benedict to ''open a history book''.
''Shall we talk about Nazism, with which the Vatican stipulated an accord a few weeks after it came to power? Or the Crusades? Or the Inquisition?'' he said.
The pope, who wrote much of Spe Salvi during his summer vacation in the Italian Alps, is believed to be working on a third encyclical which will deal with social questions.
