He blamed atheist “neo-Darwinians” for Europe’s low birth rate and said religious people of all denominations are more likely to have large families.
The Chief Rabbi, who entered the House of Lords last week, made his comments in a lecture on religion in the 21st century hosted by Theos, the public theology think-tank, on Wednesday night.
Lord Sacks said that faith had survived so far because it could provide answers to mankind’s eternal search for meaning in life - unlike the market, the state, science or philosophy, which underpin modern liberal democracies.
He claimed religion could continue to play an important role worldwide in the future, by engaging in debate with scientists, by campaigning on issues such as global poverty or the environment, and by discussing the nature of the common good with humanists.
The Chief Rabbi warned that secular Europe is at risk, however, because its moral relativism can easily be defeated by fundamentalists.
And he claimed that its population is also in decline, compared with every other part of the world, because non-believers lack shared values of family and community that religions have.
Lord Sacks said: “Parenthood involves massive sacrifice of money, attention, time and emotional energy.
“Where today in European culture with its consumerism and instant gratification – because you’re worth it – where will you find space for the concept of sacrifice for the sake of generations not yet born?
“Europe, at least the indigenous population of Europe, is dying.”
“That is one of the unsayable truths of our time. We are undergoing the moral equivalent of climate change and no one is talking about it.
“Albert Camus once said, 'The only serious philosophical question is why should I not commit suicide?’.
“I think he was wrong. The only serious philosophical question is, why should I have a child? Our culture is not giving an easy answer to that question.”
He added: “Wherever you turn today - Jewish, Christian or Muslim - the more religious the community, the larger on average are their families.
“The major assault on religion today comes from the neo-Darwinians.’’
Discussing the popular secular idea that there are no absolute moral values, he said: “You cannot defend a civilisation on the basis of moral relativism.
“In a head-to-head contest between a moral relativist and a fundamentalist, who wins? The fundamentalist must win because he is sure he’s right, and you are not sure he’s wrong.”
He said that although the war on terror had been portrayed by Western politicians as a “battle of ideas”, there is little hope that Islamists who believe they owe allegiance to God would be swayed by talk of freedom or democracy.
“The place for religion is in civil society, where it achieves many things essential to liberal democratic freedom. It sanctifies marriage and the family and the obligations of parenthood, and it safeguards the non-relativist moral principles on which Western freedom is based.
“It may not be religion that is dying, it may be liberal democratic Europe that is in danger, demographically and in its ability to defend its own values.”
Lord Sacks, who has been Chief Rabbi of the United Hebrew Congregations of the Commonwealth since 1991, described the modern phenomenon of “angry atheists” as the “intellectual equivalent of road rage”.
He said that more respectful dialogue is needed between religious groups and the secular world.
“All peace depends on compromise and that is why peace comes to seem to some religious groups to be a form of betrayal, and that is why peacemakers get assassinated.”