My father would carry me, still half asleep and clutching my toy rabbit, from the car into the Exclusive Brethren meeting room every Sunday at 5.30am. Other Brethren parents ushered sleepy children through the dawn into a windowless building in Brighton, girls with waist-length hair wearing headscarves like me and boys carrying Bibles.

We sat through four hour-long meetings on a Sunday and one every day in the week. My brothers and I would sit at the back with the women, while the men with microphones, including my father and grandfather, preached at the front. By the time I was six years old, I'd sat through around 3,000 hours of Bible study.

If you've been born into a group like the Brethren, and everyone you know is living by Brethren rules, you assume it's completely normal. It's the people outside your assembly who seem strange. Years after my family had left the group, I sometimes tried to explain to friends who the Exclusive Brethren were. People were always fascinated and they'd ask endless questions; I'd find myself saying, 'I was born into a cult,' and then recoil at the melodrama of that sentence. I was embarrassed to admit I'd been in a cult, even if I hadn't actually chosen to join it (my family had been part of the Brethren for 100 years). I knew it made me strange, serious, different. I was certain I'd never be able to explain how confusing it had been.

By the time I was six years old, I'd sat through around 3,000 hours of Bible study.

There are 45,000 Exclusive Brethren members worldwide, with 16,000 in the UK. They now call themselves the Plymouth Brethren Christian Church. Just as we did, they keep separate from the world because they believe it's run by the devil; they stick to a rigid set of rules and codes of behaviour so they can prepare for the Rapture, when they believe they will be lifted from Earth before it plunges into chaos. They don't recruit people; they have big families and send their children to Brethren schools to minimise their contact with the outside world. When I was growing up in the Sixties, we went to ordinary schools, but we were told to separate ourselves and not to talk to the 'worldly' children.

Cults aren't always religious or political groups. According to the Cult Information Centre (CIC), there are between 500 and 1,000 cults in the UK today. When I speak to Dr Alexandra Stein, an expert on cults (in the Eighties, she spent a decade in a US political cult known as The O) and author of Terror, Love and Brainwashing, she says this is likely to be a gross underestimate: 'Unless ex-cult members speak out about their experiences, the number of cults is going to keep growing.' There are yoga cults, new-age cults, eating and health cults. They share the same features: charismatic leaders enforcing an absolutist belief system, isolating their members from the world, creating a surveillance system where everyone is watching everything and living in terror of being expelled. An abusive relationship can work just like a cult, too, according to Dr Stein. Abusers isolate their victims, impose rules, undermine their confidence and keep them in a state of low-level fear. Cults can be huge, such as Stalinism, or as small as two people.

There were 300 people in the Brighton assembly when I was growing up. We had virtually no contact with people outside the Exclusive Brethren, and there were rules for every part of our lives. They told us that non-members were being controlled by Satan via radios, newspapers, TV and pop music, all of which were banned. Our mornings began with prayer and Bible reading; I learned to read with the Bible as most books were prohibited. Brethren girls and women wore hats, ribbons or headscarves to demonstrate their subjection to men and God; they were also not supposed to cut their hair and they had to wear it down their backs. I remember the feel of the headscarf knot pressing at the back of my neck and the weight of my uncut hair hanging down my back in braided ropes.

Between meetings and readings, we ran around, laughed and played games just like other children, but I often seethed with rage and frustration. Would I have to get married and be silently obedient like the Brethren women? Why wasn't anyone asking any questions? I didn't dare ask. That would get me in big trouble.

At school, we weren't supposed to talk to the non-Brethren children, but I wouldn't have known how to. When they talked about TV and football, I didn't know what they were talking about. The teachers took me out of the classroom during assembly or certain science lessons the Brethren had banned. They put us in the corridor and gave us worksheets. They didn't ask us what we were being taught at home; they probably thought they were respecting religious differences.

There were rules for every part of our lives. They told us that non-members were being controlled by Satan via radios, newspapers, TV and pop music, all of which were banned.

Why didn't I run away? Like most Brethren children, I loved my parents and I was scared of life outside. I used to hear the sound of Satan's hooves on the cobbles of our street or in the bushes of our garden, waiting to snatch us. Grown-ups were terrified of being thrown out, too. If someone was discovered with a radio, didn't give enough money to the collection or asked questions about the rules, they would be kicked out and they wouldn't see their family again. There were forced confessions, breakdowns and suicides.

I knew I wasn't going to be saved in the Rapture because of the questions that constantly spun around my head: if radios were wicked, then why did my father keep one hidden in the back of his car and listen to the cricket on it? Did the Lord know about that? Was I supposed to denounce my father for the radio?

I worried about what I would do when the grown-ups disappeared in the Rapture. How would we get to high ground before the tidal waves came? How would we stay away from Satan's people? I began to steal and hide tins of corned beef and baked beans under my bed so I'd be ready for when the grown-ups would be taken up into the sky, leaving us to face the terrors of Armageddon.

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When I was eight, my family left the Brethren. The elderly Brethren leader, called the 'Man of God', was found in bed with one of the married sisters, so 8,000 Brethren around the world, including my immediate family, left and formed a splinter group. Eventually, my parents left altogether and tried other churches. My father, at first anguished, soon discovered the joy of theatre, literature and music; he brought home a TV and took us to the cinema to see [a re-release of] Gone with the Wind. I remember stepping across the threshold of the cinema holding my mother's hand, certain the sky was going to fall.

People say it must have been amazing to suddenly have freedom. For me, it was confusing. The grown-ups never explained why we left. No one told us they'd been wrong, that they'd made mistakes and that the Rapture probably wasn't coming. They were too confused to explain, so we had to figure it out for ourselves.

When I was 12, my parents separated. My mother worked several jobs and took in lodgers to keep food on the table. I stopped talking to my father – I was furious with him for leaving us and not explaining anything. I found my way back to him when I was an awkward, bookish 14-year-old and he was addicted to gambling. He smuggled me into casinos on my school lunch breaks by bribing the doorman.

The grown-ups never explained why we left. No one told us they'd been wrong, that they'd made mistakes and that the Rapture probably wasn't coming. They were too confused to explain, so we had to figure it out for ourselves.

When I was 16 – the year before my father was arrested and sent to prison for fraud and embezzlement – he drove me all over the country to see plays. On those long car journeys we'd recite all the poetry we'd memorised, play the music we loved, argue about God, morality and Shakespeare. I finally got to ask him questions about the Brethren, but he was still struggling to understand what had happened, furious with himself for not having seen through it all, so he'd recite a poem by Yeats or Auden. I began to understand the world, and my family's history, for the first time.

When I went to university, I was forced to explain my early life with the Brethren so that certain things about me made sense to my friends. The fact that I'd stolen books from the school library when I was five, or that I had a compulsion to rescue people because I'd had dreams about being rescued myself. It's the reason I had a son in my second year of university – I wanted to start a family of my own that was safe.

Growing up with the Brethren has made me sceptical of anyone who claims to have absolute answers, or of systems with absolute rules. Even now, I still have to remind myself that time isn't about to run out, that I'm not being watched and that the sky is not about to fall in on me. I've heard other ex-Brethren describe these feelings, too. I live in houses built on hills and I avoid watching TV footage of flooding; they remind me of the apocalyptic images the Brethren instilled in us.

Even now, I still have to remind myself that time isn't about to run out, that I'm not being watched and that the sky is not about to fall in on me.

I encourage my three children to think for themselves, to ask questions and to stand up when they see something unfair. According to Dr Stein, young people are particularly susceptible to cult recruitment. They might be living in a city away from home; many are lonely. They're looking for friends, family and structure – something that makes them feel secure.

My father died before he had answers to the questions that tormented him for decades: how did a group of decent, godly people turn into a cult? How did he get duped like that? In his last days, I promised him that I'd try to figure out our shared history, take his unfinished memoir and write my own book. I owed it to him, to my own children and to all the women currently living in a cult against their will to show that it's OK to talk. But I also owed it to the small girl in the headscarf with the plaited hair and endless questions she could never ask. Only after trying to find those answers have I been able to draw a line under my childhood and find closure.

Rebecca Stott is winner of the Costa Biography Award. In the Days of Rain (4th Estate, RRP £16.99), is out now.

This article originally appeared in the July 2017 issue of ELLE UK