Sacks, in a lecture to a religious think-tank, Theos, said European secularism makes fundamentalism stronger, The Times of London reported.
A return to the Judeo-Christian tradition of religion willing to accommodate differences is "the only strong enough defense with some of the religiosity that is coming our way with the force of a hurricane," he said.
Sacks, 61, heads the United Synagogue, the largest Jewish congregation in Britain. He has been honored with the title Baron Sacks of Aldgate.
Comparing Europe to classical Greece during its decline, Sacks said secularism encourages a quest for gratification and discourages people from making the sacrifices needed to be parents. Europe is both the most secular region in the world and the only one with a shrinking population, he added.
"That is where Europe is today," he said. "That is one of the un-sayable truths of our time. We are undergoing the moral equivalent of climate change and no one is talking about it."