It was, he tells me, “the wrong question”, boiling down to whether you believe or not in “the Old Fellow in the Sky”. For more than 50 years, he has striven for approach to Judaism for those who, like him, have a strong religious sensibility but no longer accept the notion of “the Old Fellow in the Sky”.

The rector of a non-denominational rabbinical school, at Hebrew College in Boston, is widely recognised as one of contemporary Judaism’s foremost theologians. Over here last weekend to speak at the New North London Synagogue, he will be back shortly to appear at the Limmud conference in Birmingham.

Now 75, he became hooked on synagogue and ritual at an early age to “the great dismay” of his atheist father but by the end of his teens, he had lost belief in the God of the siddur, partly as a result of his grandfather’s discovery of what happened to relatives in the Holocaust.

He might have wandered away from Judaism altogether had it not been for his encounter with Kabbalah and Chasidism at university. His cheder education and his own religious zeal had given him enough Hebrew to read S.Y. Agnon’s tale of a mystical journey to Israel, Bilvav Yamim, in the original at the age of 16.

At Brandeis, the Jewish-founded university in Boston, he was taught by Alexander Altmann, newly arrived from Manchester. “He taught a brilliant course on classical Judaism,” he recalls. “He wanted the job at Brandeis, this was his trial year. All semester, he taught with bare head even though he had been an Orthodox rabbi in England because he knew that was the university game.

“One day he comes into class and I see him pacing more nervously than usual at the front of the room. The door to the classroom had a window and he peers out, slips a kippah from his pocket and reads a passage from the Zohar [the central kabbalistic text). Then he takes the kippah off and puts it away. I understood — this book was so holy, he couldn’t possibly read it without his head covered. It made a very great impression on us.”

Altmann gave the first course on mysticism at an American university. Around the same time, aged 20, Rabbi Green read the essay on the principles of Chasidism by Hillel Zeitlin (who perished in the Warsaw ghetto). “It wasn’t about God gave the Torah and that’s the answer,” he says, “It was, there is some mysterious essence to the world that unifies and illumines it. It was the pursuit of that truth and that language that struck me and later on, I came to realise this was a sort of monist theology — not just that there was one God, but there was only One and it’s about the oneness of Being and a Judaism that uncovers the secret of oneness of Being.”

His 2010 book, Radical Judaism, defines our awareness of the sacred as “an inward, mysterious sense of awesome presence”. It may come upon us in moments of great beauty, witnessing a mountain-top panorama, for example, or a sunset over the sea. The purpose of prayer and ritual is to remember those moments of transcendence when “the mask of ordinariness” falls away.

For some, such experiences may be just be a matter of excited neurons. “It is brain chemistry,” he says, “but not mere brain chemistry. We are boxes of chemicals, but we are more than that. It is a transcendent mystery we confront that our brain chemistry allows us to be open to.”

Over the years,“one of the things that has troubled me is how many Jewish seekers there are, people who were born Jews and who really care about spiritual questions, that are turning elsewhere, especially to Buddhism.”

To reach them, he has sought a “religious language that both inspires and feels intellectually honest to us as postmodern people. I turned to the Jewish mystical tradition because it allowed me to do that.”

In between his UK trips, he has visited Israel to launch the revised, Hebrew version of Radical Judaism. Due out next year are a two-volume essay collection he has jointly edited on neo-Chasidism — which includes a modern view of such topics as halachah, the environment and psychotherapy — as well as his translation of his favourite Chasidic master, the Meor Einayim (“Light of the Eyes”), Rabbi Menachem Twersky of Chernobyl.

One of the founders of the independent chavurah movement in the 1960s in America, he is only too aware that in Judaism, as other faiths, it is the more conservative forces which appear to be gaining ground. Assimilation continues apace — “if I look at the grandchildren of my first cousins, most will not be Jews.”

But he also sees “great pockets of energy” within Judaism’s liberal movements and more adults “who want to learn that I ever saw before”.

The best of Jewish tradition is what the classical sage Ben Azzai tells Rabbi Akiva: “the greatest most fundamental teaching of Torah is that every human being is the image of God. That’s taken right out of the Talmud, klal gadol, the most important rule of Judaism. To betray that for a kind of narrow exclusivism is a betrayal of what the best of Torah has to offer.”