Pope Francis: A Man of His Word is based on four interviews, each lasting two hours, during which Wenders sat face to face with the Pope. He says he asked him 55 questions. News footage from Vatican TV shows Francis the celebrity pope facing a sea of mobile phones; Francis the champion of interfaith alliances meeting Sunni clerics or praying with rabbis at the Holocaust museum; or Francis the friend of the poor, speaking to refugees on Lampedusa​.

Mostly, however, he talks. Some of what he says is quite startling, at least to an outsider like me. Railing against the growing disparities of wealth, he criticises his own fiefdom. "As long as there is a church placing its hope in wealth, it may be an NGO for charity or culture but it is not the church of Jesus," he says. "Who is the poorest of the poor?" he adds a few minutes later. "Mother Earth. We have plundered her."

Wenders is not a Catholic, having rejected the faith of his childhood when he was a student. "In 1968 I was a socialist student," he says. "I was far away from the church and took a huge detour through the early '70s, all the ideologies we went through and eastern religions until I came back to my belief, but as a Protestant." So why him? "That was on my mind as I travelled there," he says. The Vatican prefect of communications told him he used to run a film club when he was a student in Rome and that Wenders had once come to speak. "He said 'it's because of your movies'."

The Vatican, he told Wenders, would not have any hand in the film; it was for Wenders to decide what to do and raise the finance for it. All they were offering was to make the Pope available. In the absence of any parameters, Wenders says, it took him years to come up with a treatment. What he decided finally was that the film he made should be "poor cinema", shot on a minimal budget. "You can't make a film about someone who is preaching poverty if you make a luxurious production." Footage of the Pope's travels would come from the Vatican newsreels; there would be no money for a film crew to travel. His one extravagance would be short dramatic segments about St Francis, set in the 13th century; shot in black and white, they look like forgotten fragments of Bergman's Seventh Seal.

What is missing is the past. Wenders has chosen barely to touch on Jorge Mario Bergoglio's​ life as a priest in Argentina or how he came to regard climate change – as opposed to sin or the threat of hell – as his central concern. "I didn't want to get into biography," says Wenders. "I thought: 'this is a modest man: he doesn't like to speak about himself.'

"And I realised that for myself, I don't like this 'people' culture where everything we read is gossip and things about people's lives. There are books and there are going to be other TV things that talk about how he is the son of immigrants, that he is from a poor family and how many brothers and sisters he had and that he was operated on when he was a young man and that he lost a lung. I didn't want to get into that."

Nor is there any sense of what a divisive appointment he is, publicly accused by conservative Catholics of spreading heresies; the Pope has said he does not read their websites "for the sake of my mental health". Less than a week after Wenders' film had its out-of-competition premiere in Cannes, a gay man who was abused by priests as a child said the Pope told him privately that God had made him gay "and loves you like that", making headlines around the world. They are not the kinds of headlines his opponents want to see, but polls of ordinary Catholics show his flock supports him.

Pope Francis has not seem the film he helped Wenders to make. "The first thing he said when we first talked to each other was – we only had five minutes and then we started – 'you must know this, that I have heard a lot about you but I have never seen any of your films and you must know I don't know much about movies at all. It's not my thing.' So that was clear." He later sent a message to say he was happy the film was going to Cannes, but that wasn't his thing either.

You can't make a film about someone who is preaching poverty if you make a luxurious production.

It was Wenders who emerged from the experienced changed, he says. Not converted, but finding that he was thinking differently about issues of inequality and exclusion. What is dispiriting, he says, is that more people are poor than ever before. "At the point where we entered the 21st century, we thought humanity was on the right path. And now, nearly 20 years later, it seems constitutionally guaranteed, self-evident things like equality, fraternity, these simple things that we took for granted, are down the drain," he says.

"Everybody is into growth – and growth means people fall along on the wayside. The Pope's message is so simple. It is that we are equal and we should wait a little bit without growth and use less to let them catch up with us. … It takes somebody like the Pope to remind us, not just Christians, that we need to have a moral revolution happening to be able to be a community again. Otherwise we're f---ed."

Pope Francis: A Man of His Word will be screened at the State Theatre, Sydney, on June 10 and the Hayden Orpheum, Cremorne, on June 16 as part of the Sydney Film Festival.