For a minute, Maryam thought the game was up. Home from university for the weekend, she overheard her father speaking loudly on the phone. He was emotional, angry: “She’s going out drinking and getting drunk and not returning anyone’s calls,” he raged.

Maryam froze, panicked. How did he know about her drinking? And what about her boyfriend, Michael? Did he know about him, too? “I thought he was talking about me,” she says, before explaining that it was actually her cousin Nasreen who was the object of her father’s outrage.

Maryam is a 25-year-old from the north of England. She's currently at university, training to be a doctor. I recently interviewed her as part of a three-year research study, to be published later this year, on so-called “apostates” from Islam – men and women who used to follow Islam or identify as Muslim, but who no longer do.

Just over half of the 35 ex-Muslims I interviewed are like Maryam: in the closet about their apostasy, fearful that if they “come out” and open up about their disbelief that their families will reject and disown them.

Apostasy is a sin in Islam. The Quran, though it doesn’t mandate a worldly punishment for apostasy, threatens eternal torture and damnation for Muslims who leave the faith. The four leading classical schools of Islamic law on which the sharia is based – the Shafi, Hanbali, Maliki and Hanafi – go even further, stipulating that the punishment for unrepentant apostasy is death.

Although official proceedings against apostates are rare – the sentencing to death of Meriam Ibrahim this year in Sudan for allegedly leaving Islam and marrying a Christian man was a notable exception – apostasy is punishable by death in Afghanistan, Comoros, Iran, Mauritania, Pakistan, Saudi Arabia, Sudan and Yemen. It’s also illegal in Jordan, Kuwait, Malaysia, the Maldives, Qatar and Oman.

In the West, freedom of religion – including the freedom to leave a religion – is a foundational principle and is inscribed in Article 18 of the 1948 Universal Declaration of Human Rights. However, apostasy – despite the efforts of moderate Muslims – remains a stigma within Western Muslim communities.

As one British-Pakistani ex-Muslim put it: “To [Muslims], it doesn’t really matter if you do the praying stuff, because [if you're not praying] it seems that you just don’t care. But it’s a big deal if you say, ‘Yeah, I’m not a Muslim.’ It changes nothing in your actions or in what you do, but to them it means everything. Because it’s an attack on their life. It’s not, ‘Oh, he’s just a bad Muslim’ kind of thing. It’s like, ‘Shit, he doesn’t believe.’”

Tasnim, a British-Bengali woman I interviewed a stone’s throw away from the East London Mosque, told me she became an atheist at 16, but kept this secret from her family. She remembers telling a handful of Muslim friends about it at the time, but following their negative reactions she’s now careful about who she tells. She doesn’t want the “filthy looks”. Plus, people talk, and she can’t risk rumours getting back to her family.

Tasnim is now 23 and her family is still in the dark about her atheism. It’s a secret she guards closely, which involves plenty of pretence. She eats bacon and pork, but never at home or in her local area, because the consumption of pig meat is forbidden in Islam, and “you never know who’s around”.

She pretends to fast during Ramadan, and she fakes prayer: “I pretend to pray when it’s Ramadan. If I hear someone coming I quickly go to the prayer mat and pretend to continue praying,” she said. “It does feel ridiculous, but I know if I don’t pray I’d hear the whole, ‘Why aren’t you praying?’ I think it’s become easier now than it used to be. I don’t know any other life.”

Tasnim was referring to her depression, which used to be worse but has moderated over the last few years. She relates this directly to her loss of faith and the loneliness of living in the closet as an atheist in a predominantly Muslim community. “Leaving your faith,” she told me, “is the worst thing you can do, and I knew [my family] would never understand it. So there was a lot of guilt and shame and self-hate.”

She recalls feeling “lost, as though I don’t know who the fuck I am any more”. And through it all there was no one she could turn to for support, least of all her family. “It was rough, and I could barely pull myself together. I was crying 15 times a day. I could barely get out of bed in the morning. I removed myself from them, because I knew they wouldn’t accept me.”

I asked Tasmin how she felt about the prospect of an arranged marriage. Her family is already screening marital candidates for her older sister and it won’t be long before they start asking her to think about potential partners. She confided to me that she’d once considered the possibility of entering a marriage with a gay Muslim man so they could both lead double-lives, though she’s since discarded this idea. “In Asian culture [in London], everybody knows everybody, so they’d know and we wouldn’t be able to live separate lives, so we’d probably have to move away,” she said.

Tasmin is now resigned to the prospect of an arranged Muslim marriage, because the alternative – coming out as an atheist and risking losing her family – is unthinkable. “I’m grounded with them, and if I don’t have them I have nothing.”

This is Aisha’s fear, too: that if she discloses her apostasy to her family she’ll lose them. And Aisha, who was born in Saudi Arabia and whose mother follows the strict Wahhabi version of Islam dominant there, has good reason to be fearful; she’s already told her mother about her atheism, and it didn’t go well. “I just poured my heart out and said, 'I don’t believe in God,'” she told me. “And I told her that I didn’t want to wear the hijab. That I just wanted to completely disassociate myself from Islam.”

“She was hysterical,” Aisha says of her mother’s response. There was a lot of shouting and crying. Then, later, according to Aisha, her mother said, calmly: “Well, you can leave the religion, but it would mean losing us.” She also told Aisha, as she remembers it, that “if you decide to come out and tell everyone about it then you had better face the consequences, because the ruling on apostasy in sharia is death. If anyone decides to carry that out, I won’t stop them."

"So it really hurt," she told me. "I didn’t imagine my mum would say that. I know she’s really religious, but she’s my mother, too.”

Aisha’s departure from the closet was short-lived, and within weeks she had taken it all back. “I felt really guilty seeing my mum in so much pain. So I sat her down again and said, ‘You know what, you actually have a point – I don’t know enough about Islam, I think I should look more into it.’ So she took me to see the imam, and then later we did umrah [the pilgrimage to Mecca] together. I pretended I was convinced, but it really didn’t make any difference to me. I just thought, ‘I’ll keep pretending until I’m independent, and then I don’t know what I’m going to do.’”

It could be worse. Aisha’s studying physics at one of the capital’s leading universities and lives in a student dorm rather than with her parents – though she has to check in with her mother every day after lectures. “I call her before I go out and say I’m getting an early night or I’m tired. I don’t like lying to her, but there’s no other way. I can’t just sacrifice that part of my life for her.”

Aisha has also discarded her headscarf – yet another secret she’s keeping from her mother. “She wears a scarf and expects me to as well. Not just a scarf, the full body gear. I have to wear that when she’s around.”

Hanif, a man in his mid-twenties, has found the process much harder, thanks to the fact he’s done exactly what Tasmin and Aisha are trying to avoid. “I kind of let myself get pressured into getting married,” he told me. “I knew it was something my parents really wanted, and after a lot of nagging I gave in and said fine.” His wife, a woman from a tribal area in Pakistan who came to the UK to marry Hanif, is a devout Muslim.

Hanif said he first had doubts about Islam in his late teens, but never confided in anyone about it. “It was something entirely personal to me,” he said. “I felt that if I said something it would provoke an overreaction, so I couldn’t really tell anyone.”

Hanif is now a staunch atheist, but no one in his family knows – not even his wife (though “she knows that I’ve got no interest in religion”) – as he’s only told his non-Muslim friends. “I think the only thing that stops me from coming out to everyone,” he confided, “is my parents. If my parents weren’t around – if it was just my sisters – I think I’d just tell everyone. My biggest fear is upsetting my parents.”

Hanif worries that if his parents found out about his apostasy they’d berate themselves for failing to raise a “good Muslim son”.

I asked Hanif about his marriage – how he can conceal such a huge secret from his wife. “It is difficult,” he explained. “We’re not on the same wavelength on the whole religion issue, so that creates problems.”

And then there’s his love of food and drink, especially wine and Italian hams. “I have a big passion for food and wine and alcohol. I really know about my food, I know about my wines, I know about my beers,” he told me. Of course, Hanif can’t share any of this with his wife, which frustrates and depresses him, because he wants to be true to himself but feels he can’t.

He also worries about the future and how the choices he’s made are slowly sabotaging it: “I do feel like I’m wasting my twenties a lot of the time. That’s probably one of my biggest concerns in this whole not coming out thing. For example, all my [non-Muslim] mates went on holiday to Paris last year and I couldn’t go, and I was thinking, ‘What am I doing?’ It’s little things like that which make me think I might be wasting the best years of my life. My twenties should be the best years of my life.”

Author Irshad Manji with Salman Rushdie

Among liberal political commentators there’s a consensus that Islam needs a reformation to bring it into line with contemporary democratic values. Irshad Manji – the Canadian author of the bestselling 2004 book The Trouble with Islam Today: A Muslim’s Call for Reform in Her Faith – passionately argues that Muslims themselves must drive this and defend the progressive elements in Islamic Holy Scripture.

“We can draw inspiration from our own scripture, the Quran, in order to reform our hearts, our spirits and our beings,” said Manji, addressing a packed audience at the Oxford Union last year.

But with political Islamism currently on the rise and moderate Muslim voices marginalised, this may take a while. In the meantime, the more immediate problem is how to change attitudes about freedom, duty and honour within Muslim families. Until these shift, Maryam and others like her will continue to hide behind the veil of the closet, worrying about blowing their cover and all the heartbreak and shame this could bring.

All names of ex-Muslim interviewees have been changed.

Dr Simon Cottee is a senior lecturer in criminology at Kent University. He has conducted his research with the support of a grant from the Economic and Social Research Council. His book The Apostates: When Muslims Leave Islam is out in November, published by Hurst & Co.

Roberto Dassoni is a filmmaker and photographer.