Tal Keinan is a high-flier in more ways than one. He gave up his place at one of America’s most prestigious universities, Georgetown in Washington, to make aliyah. Then, virtually unheard of for a new diaspora immigrant, he soared to one of the most highly prized places in the Israeli military, in the cockpit of an F-16 jet.

The fighter pilot went on to co-found an international assets management company, Clarity Capital, with branches in Tel Aviv and New York, chairs a non-profit organisation which has distributed £250 million of loans to small Israeli businesses and helped start a school for disadvantaged children in Israel.

Now his name has become more widely known as the author of God Is In The Crowd, published last year — a “profound” and “enthralling” read, said Emeritus Chief Rabbi Lords Sacks. Part spiritual autobiography, part manifesto, it is a global call for Jewish action to arrest what Keinan fears may be the irreversible decline of the Jewish people.

He was in London last month to join Lord Sacks in the first of “Global Conversations”, a new series launched by JW3 and the Genesis Philanthropy Group to stimulate discussion on the Jewish issues that matter.

With an intermarriage rate of seven in ten among the USA’s non-Orthodox majority, the largest diaspora community, he warned, was “opting out… at a rate that within two generations there will not be a Jewish people in the US if trends remain linear”.

Lord Sacks was less anxious about the threat of extinction than by wasting the opportunity to take a Jewish message to the world, of collectively making a difference. Alluding to a song from the musical Hamilton, he professed, “I am worried we are throwing away our shot.”

Born in 1969, Keinan could well have become one of America’s vanishing Jews. His family’s religion was minimalistic; they had a Seder but ate bread on Pesach. His three elder brothers took non-Jewish wives.

His four-times married father, LeRoy Wiener, sent him to an élite New England boarding school to help him on the way to securing the American dream. As he observes in the book, “For many, Judaism was just a hindrance that they would gladly have cast off in their struggle to become Americans.”

By chance, it was the school’s Christian chaplain who presided over Shabbat dinners for Jewish students that got him thinking about his Jewish heritage. He took a trip to Israel and in Zionism found his destiny.

Over time, however, he felt Jewish nationalism was not enough on its own. Without a cohesive Jewish vision for the state, divisions in Israeli Jewish society threaten its long-term viability, he worries. “A fundamentally divided, visionless, and consequently rudderless Israel suddenly looked as vulnerable to me as the complacent American Judaism I had left behind,” he writes.

What’s more, the Chief Rabbinate’s official monopoly over Judaism in Israel is increasing the “growing” distance between the Jewish state and much of American Jewry. He and his American-born wife Amber married in a civil ceremony in the USA (the Reform chupah they had in Israel would not be recognised as a valid wedding).

In the book, he argues that Judaism evolved over time through the “wisdom of the crowd”. Diaspora communities in different parts of the world may have developed independently but often forced to migrate, they learned to reconcile their differences with other Jewish communities to find a modus vivendi. Now Judaism must continue to evolve through international dialogue, he believes.

Among his remedies are a “world Jewish endowment”, a kind of Birthright mark two but bigger. Jewish families across the world would contribute to it, in return for their children attending summer camps over two years, one in the USA and one in Israel, including participation in a “tikkun” project of social action, and additionally having their university tuition paid.

He also believes the office of Israeli president should be recast to become responsible for the basic Jewish character of the state, taking charge of Judaism’s holy places, the definition of the Law of Return and who is a Jew. Controversially, he advocates diaspora Jews being able to join in the vote for the president, who would effectively become president of world Jewry.

The book still leaves questions. On the one hand, he says, for Judaism to be relevant, it must “re-establish its spiritual value” for individual Jews. On the other, he writes, “For most of us, there is no answer in divinity. We simply do not believe in the God of scripture.”

Do there need to be Jews in the world at all, Keinan asks? While he only received one negative reply in the USA, he said at JW3, a few Israelis happily contemplated the “benign extinction” of the Jewish people.

The book is an invitation to search for an answer, for as he confesses: “I cannot put forward a compelling set of objective reasons why we must survive as a people”.