The pope intervened in the debate over the origins of the universe today by claiming that science could not explain the "ultimate meaning" of human existence.

Speaking at St Mary's University College in Strawberry Hill, south-west London, Benedict told an audience of religious leaders from different faiths that the human and natural sciences provided us with an "invaluable understanding" of aspects of our existence.

But he said science could not satisfy the "fundamental" question about why we exist.

"They cannot satisfy the deepest longings of the human heart, they cannot fully explain to us our origin and our destiny, why and for what purpose we exist, nor indeed can they provide us with an exhaustive answer to the question 'Why is there something rather than nothing?'"

He may well have been referring to Stephen Hawking, who writes in his latest book that no divine force is needed to explain why the universe was formed. He argued in The Grand Design that physics, and not a creator, was responsible for the Big Bang.

The pope warned against rejecting religion in favour of a purely scientific outlook. "Never allow yourselves to become narrow. The world needs good scientists, but a scientific outlook becomes dangerous and narrow if it ignores the riches or ethical dimensions of life. Just as religion becomes narrow if it rejects the legitimate contribution of science to our understanding of the world."

He added: "We need good historians, and philosophers and economists but if the accounts they give of human life is too narrowly focused they can lead us seriously astray."

The pope also used his wide-ranging speech – made before the revelation that five men had been arrested in connection with an alleged threat against him – to reiterate his opposition to the persecution of Christian minorities. He said that restricting religious freedom was an obstacle to interfaith dialogue.

Earlier the pope warned against the cult of celebrity and wealth, telling about 4,000 school pupils and young people that "money is not enough to make us happy".

He urged them to find happiness in God instead of modelling themselves on sport or entertainment stars.

In contrast to his strongly worded addresses yesterday, the pope struck a more informal tone. "I hope that among those of you listening to me today there are some of the future saints of the 21st century," he said.

Benedict is known for baffling lay audiences with abstruse dissertations on the faith. But on this occasion, he made a point of talking in a language that was calculated to reach his listeners.

In an address that was relayed to Catholic schools around the country, he said: "Perhaps some of you think being a saint is not for you. Let me explain what I mean. When we are young, we can usually think of people that we look up to, people we admire, people we want to be like.

"We live in a celebrity culture and young people are often encouraged to model themselves on figures from the world of sport or entertainment."

But by inviting his listeners to become saints, the pope said he was asking them "not to be content with second best".

He urged them not to be swayed by the motivations of wealth: "Having money makes it possible to be generous and do good in the world, but on its own, it is not enough to make us happy.

"Being highly skilled in some activity or profession is good, but it will not satisfy us unless we aim for something greater still. It might make us famous, but it will not make us happy."