When sitting down to interview the one of the world’s foremost religious thinkers – a sought-after voice on faith on the state of the world today – the songs of Ed Sheeran are surely among the least likely topics to crop up. Perhaps even more so when the interviewee is celebrating his 70th birthday today.

But after the best part of an hour with the then 69-year-old Lord Sacks, that’s the unexpected turn our chat takes as he discusses the lyrics of the British hit-maker’s Castle on the Hill and I get a rare insight into the down time of the man beyond the orator. “Brahms is great to run to,” he says “but so is Ed Sheeran. Michael Jackson was also pretty good on running music. My iPhone has got terrific music; music to meditate by, chill to and music to run on my treadmill to”.

I couldn’t conceal my mild amusement. But perhaps I shouldn’t have been surprised by his grasp of popular culture; his belief in a Judaism engaged with the world was after all at the very centre of his message on leaving the chief rabbinate four years ago.

Since then Lord Sacks has taken that message to communities from Mexico to South America and to the wider public, including when he became the first religious figure to take the main stage at TED’s  HQ – an experience  he describes as “ one of the most nerve-wracking in my life”.

That talk was streamed to 100,000 people in cinemas around the world and has been viewed more than 1.5m times online.

The peer has also innovated on social media, where a series of viral whiteboard animations voiced by Lord Sacks attract hundreds of thousands of views. While Lord Sacks had long planned for post-chief rabbinate life – “as I said to the government when it asked for my support for the Iraq war ‘have you got an exit plan?’” – he didn’t predict the extent of the interest in his message “We couldn’t have foreseen the impact of social media and the development of the internet. This has created possibilities that never existed before, you can function globally in a way nobody could before, immediately and at low cost. It is also down to having the greatest team in the world.”

We couldn’t have foreseen the impact of social media and the development of the internet. This has created possibilities that never existed before

But he suggests ever-developing  technology – alongside weakening families and communities beyond religious groups – is one reason for increasing numbers of people turning to political extremes. “Humans can get used to almost anything: poverty, disease, war. What they can’t get used to is change. So they search for certainty and those certainties tend to lead them to extremes. When you’re trying to live by simple truths in a complicated world, you do become extreme.”

He sees this trend continuing. “We don’t know quite what the impact of, for instance, artificial intelligence is going to be. We are living in a very uncertain world and it’s going to become more uncertain. So it’s more and more important to safeguard support networks and there’s nothing more powerful than strong communities and families.”

Lord Sacks is convinced that the religious voice is more crucial than ever. “When politics is divisive, religion has a duty to unite.”

Despite having his eyes firmly trained on the future and having “made it an absolute point of principle” not to tread on his successor’s toes, he heaps praise on Chief Rabbi Mirvis  for “creative” initiatives including Shabbat UK and education programmes for women.

Chief Rabbi Mirvis (left) with Lord Sacks (centre) and Prince Charles

He also “salutes” the Chief Rabbi’s decision to go to Limmud, despite his own decision not to. “I knew that Limmud is a fraught issue in the Jewish community, rightly or wrongly. For all my 22 years in office I said every rabbi that wanted to go should do so with my blessing. The Chief Rabbi going has not ended the controversy and I knew that my going would not end it either.”

He says restrictions are a natural by-product of leadership. “I felt privileged for the whole of my chief rabbinate. But when you’re captain of the team you have to play by the rules of the team. That means you can’t always say and do what you would do as a private individual. So for the last four and a half years being able to speak in my own voice and make my own decisions without necessarily worrying about others has been exhilarating. People have noticed. I feel a lot younger than I did five years ago.”

Lord Sacks giving his famous Ted Talk

Having overseen an explosion in Jewish school places under his chief rabbinate, education remains a key focus today – whether through the launch this week of a curriculum bringing together his teachings with classical sources or in America where leaders have asked for his help. “American Jewry will never be like Anglo-Jewry because it’s bigger and has a different political culture. But the bigger respect in which we’re different is that we’ve built day schools. In America now, outside the Orthodox community, the religion is haemorrhaging. In many ways British Jewry remains strong in a way that American Jewry has historically been strong, but today is beginning to weaken at the edges. We’re trying to tease through what would constitute a major campaign of the kind we did in Anglo-Jewry under Jewish continuity.”

“My iPhone has got terrific music; music to meditate by, chill to and music to run on my treadmill to”

Sticking with America, he said he “understood” the criticism over his key role in helping Mike Pence draft his much-lauded Knesset speech.  But Sacks said that while “they were looking at a person”, he was focused on issues. “It was the vice-president of America delivering a message on behalf of the American government about recognising Jerusalem. Whoever had been the person I would, if asked to help, have done so.”

As he hits the big 7-0, just ahead of the Jewish state, he praised the country as “young for a 70-year-old nation” that thrives “because it empowers the young”. Though the announcement last week of the first official Royal visit came as a surprise at a moment of political deadlock, he praised the “wisdom and courage” of the move during the birthday year.  Though he understood the difficulties for the British authorities, he said the Royals “had probably wanted to go before”.

In many ways British Jewry remains strong in a way that American Jewry has historically been strong, but today is beginning to weaken at the edges.

The country’s greatest challenge, he insists, remains to find a way to embrace and bring together Israelis of all religious levels. He said: “Israel’s president, a man I hugely admire, has been speaking in recent years about an Israel of four minorities: the secular, the religious, the ultra-religious and the Palestinians who do not share a common narrative. We are the world’s experts in creating narratives. Where is the vision in Israel of a society that embraces both some very holy people who are to Israel what the priesthood was in biblical Israel, plus a religious and secular public who, though they may be secular, are to a large degree very open indeed to Judaism if it is not forced on them.

Rabbi Lord Sacks receiving his lifetime achievement award from Tony Blair at Jewish News’ Night of Heroes last month

As for the birthday boy, there’s little sign of the “semi-retirement” for Lord Sacks that Prince Charles referred to back in 2013. He was recently honoured with a lifetime achievement award at Jewish News’ Night of Heroes, which he describes as “one of the best communal events I’ve ever attended because it lifted our moral and emotional horizons”.

He insisted his “young team” keeps him fresh (he wears a Fitbit on his arm to track his healthy lifestyle) and says that continually moving outside his comfort zone – including comedy video with Ashley Blaker – keeps him from going stale.

He is currently working on his chumash, “the biggest project I’ve ever undertaken”, and is promising a five-part series on BBC radio on the big moral issues facing humanity. “I’m looking after my health,” he says. “I’m exercising as hard as I can because this work needs to be done.”