Two decades ago, when I was a young, hapless academic, I agreed to speak to the Edinburgh Jewish Literary Society. I was writing a critical biography of Dame Muriel Spark at the time and so it was only natural that I would talk on one of the greatest writers that Edinburgh has produced. Little did I know what would follow.

Spark (née Camberg) was born in Edinburgh towards the end of the First World War, on February 1, 1918. Her centenary will be celebrated next week with calls in Scotland to erect statues in her honour so as to place her Scottishness in concrete. But her identity is far from straightforward.

Spark described herself as a “Gentile Jewess” or “half Jew” (with a Jewish father and Christian mother) and converted to Catholicism in 1954. Spark left Edinburgh for Rhodesia at the age of 19 and had a disastrous marriage to Sydney Oswald Spark (“SOS”). She never lived in Edinburgh again and became a self-styled “exile” residing in London, New York, Rome and, eventually, Tuscany.

Her estranged son Samuel Robin Spark (who died two years ago in Edinburgh at the age of 78) was raised by Spark’s parents, Bernard and Sarah Camberg. Spark left Rhodesia for London in 1944 (a remarkably hazardous journey) when Robin (as her son was known) was six years old. His father, who had poor mental health, obtained custody of Robin after divorcing Muriel. Robin had a barmitzvah and claimed that his beloved grandparents (who quickly became loco parentis) were “fully Jewish”.

By then, Robin was a member of the traditional congregation in Edinburgh and possessed a copy of his grandparents’ Orthodox ketubah. His grandmother, despite what Spark argued, must have been Jewish to obtain this.

I went to Edinburgh to solve a mystery: Can a convert and “gentile Jewess” be a Jewish writer? The evidence I brought was Spark’s superb story, The Gentile Jewesses, which was included in The Penguin Book of Jewish Short Stories. Spark also wrote one of the most interesting English-language novels set in Israel, called The Mandelbaum Gate (published in 1965). The novel includes a first-hand account of the Eichmann trial (witnessed by Spark during a two-month visit to Jerusalem).

At its heart is the uncertain “gentile-Jewish” identity of its Spark-like heroine, Barbara Vaughan, when compared to the certainties of the Israelis she meets.

It is the most ambitious of Spark’s 22 works of fiction. She spent two years composing The Mandelbaum Gate as compared to the eight weeks she took over her most famous work, The Prime of Miss Jean Brodie (1961). She eventually disowned the Jewish novel coming to regard it as too long and unwieldy. Given its subject matter, it is fitting that this work continued to disturb her.

Which brings me back to the Edinburgh talk. The audience at the Jewish Literary Society responded to my account of Spark’s mixed background by pointing out to this hapless academic that Spark’s parent’s had a ketubah. She was therefore “fully” Jewish. In response to this revelation, the JC carried the front-page headline, which ignited the festering differences between mother and son, “full” and “half” Jew.

This dispute went on in public for months and has been recently re-ignited in Alan Taylor’s short, gossipy Appointment in Arezzo.

But, as Barbara Vaughan says in The Mandelbaum Gate: “There’s always more to it than Jew, Gentile, half-Jew, half-Gentile. There’s the human soul, the individual. Something unique and unrepeatable”.

Such is the credo of a towering novelist, which was understandably rejected by her estranged son.

Bryan Cheyette is currently writing a short history of “the ghetto” for Oxford University Press