On one side stood the patriarchal Vatican and an austere archbishop called Cardinal James McIntyre. On the other, a group of creative and courageous nuns determined to challenge the status quo and embrace the 20th century.

There was only going to be one winner.

The story of the Sisters of the Immaculate Heart of Mary in Los Angeles, who took on the might of the Catholic church in the 1960s, is told in the new documentary Rebel Hearts, directed by Pedro Kos with a striking combination of graphics, music and archive interviews.

The film recounts how some women saw the religious order as an expression of independence and a way out of their socially expected role as suburban housewives. But they discovered the church came with some antiquated and suffocating conventions of its own.

“Many of us read a book called Asylums about mental institutions,” says one interviewee, “and when we read it, we began to realise that the same kind of restrictions that were put on people in mental institutions were also the kind of rules that controlled our lives.”

Inspired by the Second Vatican Council, a body designed to modernise the church, the sisters carried out a study of their own lives and work and decided to seek a more liberal path that included contemporary dress, flexible prayer times and expanding their ministries beyond health and education.

Unsurprisingly, they met with resistance from the rigidly conservative McIntyre, who understood the threat to his control of the women’s minds and bodies. The dispute went all the way to the Vatican which, again unsurprisingly, sided with the cardinal.

More than 325 sisters renounced their vows, broke away and formed the Immaculate Heart Community of California (still thriving after 51 years), while 65 kept their vows and remained obedient to the church in Los Angeles or transferred to other orders.

The history of Immaculate Heart is also the history of the 1960s – feminism, flower power, challenges to authority. The sisters took up causes such as civil rights and protesting against the Vietnam war. One of them, Patrice Underwood, marched with Martin Luther King across the Edmund Pettus Bridge in Selma, Alabama, in 1965.

Another heroine of the film is Corita Kent, who headed the art department at Immaculate Heart College. She was a pop artist and social justice activist whose work became increasingly political, addressing poverty, racism and war; McIntyre described it as “weird and sinister”.

Lenore Dowling was a sister who worked with Kent in the college art department, teaching film classes. “Corita was always under attack when she spoke against the Vietnam war, spoke for Martin Luther King and Robert Kennedy,” she recalls in a Zoom interview.

“The college was under the microscope of the chancery and was the object of criticism but the courageous women somehow went on and the work went on, the art went on, Mary’s Day [a joyful celebration deemed blasphemous by McIntyre] went on. There was vocal and public criticism from the cardinal but the college and faculty and students went marching on.”

Much media coverage at the time focused on whether the sisters would be allowed to ditch their nuns’ habits. But for Dowling this was something of red herring.

“I loved being a nun, I loved wearing the habit,” she says. “I think wearing the habit or not wearing the habit had to do with our choice of where to live, where to work, and the habit would be inappropriate in other venues.

“We were only teachers and nurses, so it didn’t matter in the schools or in the hospitals but after we changed and were self-determining, many of our members went on to study law, to go into the medical field, to be in parishes. So it’s more a matter of what you wear in order to go to work. The Second Vatican Council said be part of the modern world.”

Dowling, who at 90 is still politically active – she took part in a women’s march in Los Angeles in 2018 – adds: “I respect nuns who still wear the habit, and there are many in this country who still do, but their lives are convent lives.

“When you leave the convent, you leave the trappings of a more walled-in existence. To go over the wall or leave the wall or break down the wall meant that some of the practices associated with being in the convent and wearing the habit changed for us.

“We decided for ourselves where to get further training, where to work, where to live, when to pray. The habit was a disciplinary issue that the cardinal disagreed with but our issues were much bigger: are we going to be able to make choices to fulfil our mission to serve people in the world?”

The dress code controversy was a way of deflecting attention from a broader dehumanisation. McIntyre had overseen a massive school-building programme in southern California and used the sisters to staff them. They were paid very little and could face classes of 70 or 80 children.

Rosa Manriquez, 68, a member of the Immaculate Heart Community, says: “In authoritarian groups, little things are put in for people to start talking about in order to shield the big issues.

“With Cardinal McIntyre’s complaints concerning the community, there was very little attention paid to things like whether they’ll be able to choose what their work was going to be; what time they were going to pray; to be able to be a teacher or not be a teacher during the same time that they’re going to college; to withdraw from a school that the archdiocese was not funding correctly.”

When the sisters rebelled, McIntyre proved an implacable foe and many felt he mishandled the situation. Under pressure from the Vatican, he resigned in 1970 at the age of 83.

Manriquez, who was educated by Immaculate Heart sisters and blamed McIntyre’s vindictiveness for the closure of a school she attended in downtown Los Angeles, reflects: “I would say that it was necessary for me personally to forgive Cardinal McIntyre because my childhood was just ripped upside down through his actions.

“It was necessary for me to do that because feeding resentment, anger, hate, all the rest of that did more damage to me than it did to him. Simply forgiving and realising that he was not an angel, he was not a devil, he was not a saint, he was not a demon, he was a human being. And human beings can do beautiful, marvellous things and they can do very stupid things. He was simply a human being and he has his place in history and we have ours and I dare say I think things came out right.”

Half a century later, how does Dowling feel about McIntyre now? “Rather than feeling negative – and I know some of our members still have negative feelings – I think he created the opportunity for us to follow what the Vatican Council had said to be part of the modern world.

“We were at the same time following the spirit of being part of the modern world and having to resist orders from the hierarchy. I see it as a way for us to step out of past boundaries and to look for a new path to self-determination, to being able to serve in the spirit. We were called to serve others, to work with the poor, to be compassionate people and to go into the world rather than to retreat from the world.”

It is surely no coincidence that this drama unfolded in Hollywood, a place where McIntyre – a stick-in-the-mud somewhat reminiscent of Meryl Streep’s school principal in the movie Doubt – was always going to struggle to hold back the tides of change. Activists such as Jane Fonda, Tom Hayden and Buckminster Fuller turned up at the college campus.

Dowling says: “I think the climate and the culture of Los Angeles contributed to our faculty, our students, everybody’s willingness to grow in the sunshine. Physically, morally, educationally, intellectually, that was a time and it still is a time of curiosity, of experimentation, of taking risks and warming to the sun and being welcomed and welcoming others to our western oasis.”

Manriquez, a racial justice and LGBTQ+ rights activist critical of today’s Vatican, agrees that the location was key. “How are you going to keep any of this secret? An institution that thrives on secrecy was trying to do a great spiritual violence to a group of women in a city where there are no secrets.”

She adds: “I’ve heard it said that if you want to make God laugh, tell God your plans, and I get the feeling that God’s got a very perverse sense of humour to have put Cardinal McIntyre in Hollywood at the time that He or She put Cardinal McIntyre here.”

Director Kos, who grew up as a gay man in Catholic Brazil, says that when he first heard the story of the Immaculate Heart sisters from producers Shawnee Isaac-Smith and Kira Carstensen, it was like an arrow through his heart.

“The more I worked on this film, the more current the story became,” he explains on Zoom. “I never felt like I was making a historical film. I felt like I was making a film about today and really exploring and doing a deep meditation on change and what this change looked like.

“The twists and turns and the ebbs and flows and the perseverance that it requires and the toll that it can take and the sacrifices one has to make really speak to where we are now in this moment where we are having to question these authorities and these structures that govern our lives.”

• Rebel Hearts is now available on discovery+ in the US with a UK date to be announced