What makes a Jew? This is the question at the heart of Israeli filmmaker Ronit Kerstner's documentary Torn. The film, which was screened last week as part of the 10th Kinoteka Polish Film Festival in partnership with UK Jewish Film, tells the story of Romuald-Jakub Weksler-Waszkinel (now known as Yaacov Weksler-Waszkinel), a Polish Catholic priest who discovered, 12 years after his ordination, that he was in fact the son of Jews killed in the Holocaust. To ensure his safety, his mother had given him up to be looked after by a Polish couple days after his birth.

The film follows Yaacov's attempt to settle in Israel under the Law of Return. The statute does not recognise Jews practising other religions, and the Ministry of Internal Affairs rejects his application for citizenship. Instead, he is granted temporary residency on a religious worker's visa. He tries to join a kibbutz but asks to leave on Sundays to observe Mass at a nearby monastery. A compromise is found allowing him to worship alone in his room. But one kibbutznik still wants to know: "Who are you, Yaacov? Are you a Jew? Are you a Christian?"

Although the question has haunted him for most of his life, it seems clear that the gentle 69-year-old, who was in London for the screening, is a man at peace with himself, having reconciled his Christian and Jewish identities through the love of his adoptive and murdered parents. "The only reason I cannot say no to my Polish parents is their love for me," he says. "The only reason that, for the rest of my days, I am going to shout that I am Jewish, is my love for my Jewish parents. The rest is the Holocaust. My only fault is to be born at the wrong time, and to survive."

Circumstance has made him a Jew who cannot deny Jesus, but he does not feel torn. It is the world that wants to tear him apart, he insists, "because people like to have clear and strong divisions… I simply want to be both. Without my Jewish parents I would have no life. Without my Polish parents my life would have perished."

The latter were devout Catholics who showered him with love. He had little reason to think he was not their offspring, although he did sometimes wonder why he did not share their Slavic features. So did other people. "I had black curly hair and on the street they called me 'Yid'," he says. "Children would say, 'Who are your parents?'."

Without my Polish parents my life would have perished

He waited decades before asking his mother, Emilia, about his background. "I was afraid," he says. "When I went to church all I heard was bad things about Jews, like Jews murdered Christ. I didn't want to be somebody like that."

When, finally, he asked the question, Emilia could only tell him that he was born in the ghetto in Swieciany and that his father was a tailor - she did not know his name. Through a nun who met survivors from Swieciany, he later discovered that his father was called Jacob Weksler and had died in the Stutthoff concentration camp. He also found out that his mother, Batia, and his older brother, Samuel, both died at or on the transport to Sobibor. When he saw a photograph of Batia, "I found the first person who I looked similar to," he says.

There were living relatives, too - an uncle and an aunt in Israel, whom he met in an emotional reunion.

Returning to Poland, he became increasingly aware of the Catholic church's antisemitism. He felt isolated and lonely, and as his retirement as a philosophy professor at the Catholic University of Lublin approached, he decided to move to Israel.

Today he is a permanent resident there and works at Yad Vashem, which has named his Polish parents Righteous Gentiles. "Everything that I find there at Yad Vashem is about the fate of me and my parents," he says.