Standing by the crematoria at Auschwitz, Csanad Szegedi shivered with horror as he stared bleakly into a pit where the ashes of hundreds of thousands of Jews had once been so mercilessly discarded.

As a Jewish boy, trying to grapple with the magnitude of the crimes which took place on this blood-soaked soil, he felt, almost literally, as if he was gazing into the abyss.

"I thought to myself, I'm standing by the graveside of my family, of people I never knew. How could this have been allowed to happen?"

For Jews everywhere, it's an understandable reaction. Yet in the case of this 32 year-old businessman, it's also an extraordinary one.

For only three years earlier, Csanad Szegedi had no idea he was Jewish. In fact, he was the deputy leader of Hungary's far-right Jobbik Party and was known for his unapologetic position as an ultra-nationalist and staunch antisemite. In short, he loathed the Jews, his running incendiary narrative designed to provoke hatred.

My aim is to get people to know what it means to be Jewish

But in a fabled heartbeat, his life changed. Visiting his 92-year-old grandmother back in April 2012, the elderly lady made an extraordinary revelation, one which she had been concealing for more than 60 years.

Clasping her grandson's arm, she disclosed that she was in fact Jewish - and because of Judaism's matrilineal line, her daughter, Szegedi's mother, was Jewish too. This also made Szegedi a Jew.

"I'd gone to visit my grandmother one afternoon and she was gently reminiscing about the past. Then suddenly she made this astonishing confession. At first, I couldn't grasp it. Then I just broke down. I couldn't absorb what she was telling me. It didn't make sense. How could this be happening?"

But there was more. Rolling up her sleeve, Szegedi's grandmother slowly exposed a string of numbers tattooed on her arm. A heinous hallmark of time spent interned Auschwitz death camp. (His grandmother wore long-sleeved shirts or a plaster in summer, to cover the tattoo.)

"I couldn't believe it,'' recalls Szegedi. ''How could I be a Jew? I thought, this is the worst thing that could ever happen - there couldn't ever be anything worse than this."

As Jews, we often believe we semaphore our heritage to those we intuit are fellow Jews. For Szegedi, however, there had been no clues.

"Nothing could have prepared me for this, though there had been times when my mother and grandmother took issue with my antisemitism. Not because it was about Jews, they said. But because they felt it was wrong to hate anyone."

Today, Csanad Szegedi is a changed man. From railing against the Jews he has become evangelical in his mission to tell his story and warn against the toxicity of antisemitism.

He speaks to schoolchildren, students, indeed anyone who will agree to listen.

On the day of this interview he has travelled from Budapest, where he still lives, to speak to a group of Jewish professionals at an event organised by Chabad Manchester City Centre.

It's especially meaningful since it was to Chabad-Lubavitch's Hungary director, Rabbi Baruch Oberlander, that Szegedi turned for advice when he discovered he was Jewish.

"He told me to do the opposite of everything that I had done before. He said that, in the Talmud, it says that, if you become a better person through study and doing good deeds, you will cure the bad deeds you have done in the past. Repentance was the key. And he said that I should educate Jews about antisemitism, find out why people feel this way, and connect with my Judaism."

It wasn't, however, his first port of call. After discovering his grandmother's bombshell revelation, Szegedi acted with supreme loyalty to his party and went immediately to its leaders to break the news and tender his resignation. Though equally horrified, there was no knee-jerk reaction from party officials. Over the next two months, they considered keeping it a secret, or perhaps using him as a "token Jew" to deflect accusations of grass-roots antisemitism within the party,

By July 2012, he had resigned. When he broke the news to his parents they couldn't conceal their fear and dismay.

"My father said, 'why on earth do you want to be Jewish?'. I told him that it wasn't a desire, it was who I was. That it wasn't a matter of choice. There were people in Auschwitz who were murdered for being Jews. They may not have wanted to be Jews but had no choice."

(Only recently, did he confess to his father that he had been circumcised two years ago. "I waited until we were on a flight together, when he was wearing his seat-belt, so that he would have to listen," he dryly remarks.)

Junking his old life, Szegedi set fire to copies of his own biography, I Believe in The Resurrection of the Hungarian Nation, and began the journey from confirmed antisemite to someone who wanted to learn, practise and embrace his Jewish faith. Someone who - tall and a little rounded resembling Ewen MacIntosh's character, Keith Bishop, the deadpan accountant in the BBC hit comedy The Office - certainly looks Jewish.

This is all hard to square with the former reactionary who co-founded the Hungarian Guard - a paramilitary outfit that marched in uniform through Roma and Jewish neighbourhoods?

The great irony of Szegedi's dedicated antisemitism is that its genesis dates back to his grandparents' decision to conceal the fact that they were Jews.

After his grandmother's liberation from Auschwitz, she met her husband, who had been imprisoned in Nazi labour camps, and they started a new life together in their native Hungary.

Fearing antisemitism, they began to assimilate, even changing their Jewish surname to something which sounded more Hungarian. His mother, encouraged by her parents to 'marry out', met a non-Jewish boy, with whom she settled down.

"My father was a nationalist and my grandparents were quite happy. They felt they had 'made it', that they had escaped their past," Szegedi says.

Meanwhile, the atmosphere in Hungary was becoming increasingly antisemitic. Influenced by what he describes as ''an ever growing spiral of hatred'' Szegedi worked hard at becoming a racist.

"I read about it, studied it, and became one," he says simply.

In 2001, when he embarked on a degree in history at Budapest University, he met fellow students who happened to be Jews.

"And they were okay," he recalls, almost sheepishly. "We went to parties together. But antisemitism isn't about the Jewish people, it's about the antisemite. The antisemite is projecting his own fears."

These days, Szegedi is an Orthodox Jew. His wife is converting to Judaism. His sons are both circumcised. He even jokes that he has gone into real estate - "a proper Jewish profession."

It hasn't been an easy journey, On the one hand, early forays into synagogue life were greeted by unvarnished remarks about 'that Nazi'. On the other, he has himself been the butt of antisemitic comments.

Recently, he took his father to Israel - "a place I love" - visiting both the Kotel and the graveside of Rabbi Akiva. "Imagine the sight of my nationalist father praying at the graveside of one of Judaism's greatest scholars and holiest men?"

Yet he defends his people. "Hungarian people are not antisemitic, although there is an antisemitic discourse in society," he says.

And he points out that Budapest bustles with Jewish life, including kosher restaurants, synagogues, and Jewish shops. Wearing a kipah, he says, might trigger a few funny looks, "but you won't be spat on, or physically threatened as you might be in France or Belgium".

Meanwhile, his mission is unequivocal. "I want people to understand Jews. Antisemites don't know about Jews."

The world should listen. This is a man who knows about being both.