Above all, he is an optimist. He is the first to admit, for example, that his marriage wasn't perfect. Kylie suffered regular bouts of mental illness, including postnatal depression in 2002. Despite being close to her mother, who lived a short drive away in the Blue Mountains, she often felt lonely. She suffered a breakdown in 2004, then another in 2006, when she threatened self-harm, and, according to Nathan, falsely claimed to have swallowed two packets of Panadol. Hideaway … Kylie and Virginnia's rented home in the Blue Mountains. "Yet, with the help of family and friends, Kylie always got better," he says. Indeed, the couple had even decided to try for more children and had extended their mortgage to renovate the house. In April 2008, with her job at Nepean Hospital yet to start, Kylie found work on the front desk at Wycliffe Christian School, where she met an older woman called Virginnia Donges. Donges, a mother of six, had worked as the school's first aid officer, and was well known in the community. One day she invited Kylie to join a Bible study group, weekly meetings of which were held at members' houses in the Blue Mountains. "Kylie wasn't a lady who made friends easily," says Nathan, "so I was pleased." Soon the meetings began to run later and later. Kylie would often stay out past midnight, without any explanation. Sometimes she wouldn't come home at all. When Nathan tried to call her, he would receive a text message, usually from Donges, saying that Kylie would "be home in the morning".

Nathan became increasingly worried: Kylie had been anxious about her new job, and had been showing signs of another depressive episode. But he was unsure what to do. "Kylie would take our only car, and I had a five-year-old in bed," he says. "I couldn't just take off after her." Besides, he told himself that she was in good hands: he knew Donges from her work at the school, and the group's other members seemed equally reputable. On January 4, 2009, however, Kylie left for another meeting, telling Nathan she would be back for dinner. Liam and Nathan waved her off. "She never came back," he says. When Kylie had been gone for three days, Nathan, with Liam in tow, visited Donges' home in Faulconbridge, in the Blue Mountains, where she lived with her husband, Wayne. There they found Kylie sitting, according to Nathan, "mute and unresponsive", in the lounge room. "Liam ran into her arms, but she was catatonic. In the end he just played Lego at her feet for two hours." Donges, meanwhile, assumed the air of a hospital matron. "She looked at me very seriously and said, 'Nathan, you have no idea what we've been going through.' " Apparently, Donges had been "counselling" Kylie for the most horrific child abuse, memories of which had come to light only after months of gruelling "therapy". Kylie, it seemed, had been raped and subjected to ritual Satanic abuse. She had, as a small girl, been taken from her bed by hooded figures and forced to witness murders. She had had boiling water poured in her ear, and been kept under the house and fed like a dog. Not all of this was news to Nathan. Kylie had in the past spun wild stories about having participated in a murder, being raped and having an abortion. (She later retracted these claims.) But what Donges said next went one step further: as a way of coping with her abuse, Kylie had developed dissociative identity disorder (DID), and was now harbouring hundreds of separate identities, or "alters", the most dominant of which was a six-year-old girl named Hope, who sucked her thumb and had to be put to bed with a teddy.

From Dr Jekyll and Mr Hyde to the 1976 film Sybil, DID, or multiple personality disorder, has become one of the most storied and controversial of all diagnoses. Springing in part from Sigmund Freud's theory of repression, DID is characterised by the presence of two or more identities that manifest recurrently to take control of the subject's behaviour. Each identity exists in isolation and without knowledge of the other, each sequestering memories of events too painful to bear. Reports of "multiple personalities" go back centuries: such behaviour was often attributed to demonic possession. These days, many psychiatrists doubt that DID exists. Others suspect it is predominantly "iatrogenic", or brought about by its own treatment, which often comes in the form of recovered-memory therapy. "Recovered-memory therapy is when a counsellor leads the patient back to their other identity," says Don Thomson, professor of psychology at Deakin University in Melbourne. "When they arrive at that identity, they will be able to recall details of the initial trauma, which can then be properly processed." Recovered-memory therapy is even more controversial than DID. In the 1990s it was implicated in a spate of court cases, both here and overseas, where victims recalled suffering incidents of child sex abuse, usually at the hands of their parents, that were later shown to have never occurred. "It's surprisingly easy to suggest false memories," says Thomson. "There's clear evidence that people who are emotionally distressed, when placed in an environment where they feel supported, are highly suggestible." For this reason, experts recommend caution when dealing with recovered-memory therapy and DID. Yet when Nathan asked Donges how she had arrived at her diagnosis, he says she told him, "Spiritual discernment." (Neither Kylie nor Donges would be interviewed for this story.) "At that stage, I said, 'Look, Kylie has to come with me, she's sick.' But they wouldn't allow it," says Nathan. "When we left, Liam waved his arms in her face and said, 'Mummy, Mummy, please talk to me. Why won't you talk to me?' But Kylie just sat there."

Over the following year, Nathan tried everything to get Kylie back; he called, he emailed, he wrote letters. He visited Donges's home, into which Kylie had moved. He went to the police, who told him they were powerless. He consulted Kylie's parents and siblings, whom she had also disowned. (Kylie was supposed to be a maid of honour at the wedding of her younger sister, Briony, in February 2009, but never showed up.) On Christmas Day, 2009, Nathan called Kylie and pleaded with her. "I said, 'Where are you? We miss you.' She said, 'I'm with my real family now.' " Kylie had long since lost her job at Nepean Hospital and assumed the name Hope - that of her six-year-old "alter". She had also been seen by a Blue Mountains psychiatrist, who after just two sessions confirmed a diagnosis of DID. In 2010, Kylie signed an enduring power of guardianship, giving the "Bible study group" the right to make medical decisions on her behalf. Her contact with Liam, meanwhile, was sporadic. Once, at a meeting mediated by Nathan's mother, Helen, Kylie turned up with another member of the group, who instructed Kylie to manifest one of her "alters", which they called "The Dark One". "It was creepy, like some bad B-grade horror movie," says Helen. "Liam was extremely upset." (Liam has been in Nathan's sole care since February 2011.) Any spare time Nathan had was spent looking into the Bible study group, the name of which, he discovered, was the Springwood Faulconbridge Home Group. Small, amorphous and without any fixed place of worship, the group is what is known as a "house church", one that is thought to have begun 20 years ago. "In the beginning, it was all pretty normal," says Blue Mountains-based Wendy Morton, who with her then husband and two children attended meetings in the mid-1990s. (Donges was not yet a member, but other current members were.)

"Back then [the church] didn't even have a name, it didn't need one. It was just a small group of like-minded Christians who would meet on a Sunday afternoon, study the Bible, sing a few songs, then maybe have a meal together." In 1998, however, one of the group became ill and ended up in hospital. "We all thought he was going to die," says Morton, "and so we started praying for him." At some stage, the nature of the prayers "turned strange", according to Morton. "One of the group [who has since left] began acting as though he was getting special messages from God, which he had to impart to the group. He would also say, 'Me' when relaying these messages, as if he were God. He also began making predictions about the return of Christ, and saying that we would each have privileged roles in His return." One person was to be "The Arm of Comfort" (Morton had no idea what this meant); another would be "an Aaron-like figure", referring to Aaron the Priest, who in the Hebrew Bible is the brother of Moses and a prophet of God. Morton strongly disagreed with the direction the church was taking. "I just didn't believe that a small group of Christians in the Blue Mountains had a special dispensation for the End Times." But when she raised her concerns, she says the other members became "very hostile and quite confrontational".

Morton left in 1999, but now believes there is a direct link between their beliefs then and "what is going on now, which is just really unhealthy". Just how unhealthy became apparent in August 2011, when Nathan was contacted by Virginnia's husband, Wayne. Wayne had initially been a member of the group but began voicing concerns in late 2009, whereupon Virginnia abandoned him and their teenage daughter and moved with Kylie into a rented house nearby. When Virginnia moved out, she left behind notes and papers that Wayne subsequently collected and passed on to Nathan. Written by a variety of people, the papers are at once bizarre, comic and sinister. Some appear to document Kylie's "treatment" - regression sessions, mainly, where she would relive childhood traumas and write in an infantile scrawl details of unimaginable abuse, including being showered in a purée of human body parts, being sexually assaulted and forced to drink blood. The documents also indicate that the group believed themselves to be engaged in spiritual warfare with witches - both in the Blue Mountains and in Sydney - some of whom are identified by their name, age, employment and "coven status". "One of them is my aunt, who lives in Harbord," says Nathan. The documents also confirm the group's involvement with a man named John Darnell, who is the pastor of a church in Canberra called Shepherd's Heart. Darnell is the author of Satanic Strategies in the 21st Century, which explains how Satanic cults have infiltrated the "highest echelons" of government and mainstream churches. On a US Christian radio station last year, Darnell said he believed it was possible that the British royal family were actually shape-shifting reptiles.

A large part of Darnell's work involves DID, which he believes is a function of demonic possession. "Under trauma, humans have the capacity to reproduce themselves, to split into different identities, and Satan can sometimes get control of those separate parts," he tells me. In his Ministering to Dissociation Course Manual, Darnell borrows heavily from medical texts - there is much mention of the prefrontal cortex and "myelination of the hippocampus" - but soon lurches into talk of demons and ritual abuse, and the role of Jesus in cleansing the "client's" spirit. Kylie underwent such a treatment at Shepherd's Heart in Canberra in 2010, although Darnell downplays it. "I just loved and helped and prayed with her," he says. When talk turns to Nathan, Darnell is scathing. "The headline Nathan wants is, 'Wife leaves for Bible study group and never comes back.' That's bullshit. The real headline should be, 'Desperately unhappy wife leaves husband and husband refuses to accept it.' " Kylie is the eldest of four children, only one of whom, Briony, would be interviewed for this story. "Mum and Dad are devastated," she tells me. "They don't want to talk."

When Kylie first took off, her family tried to contact her. "Dad went to the house in the Blue Mountains," says Briony, "but Virginnia was quite abusive towards him. She wouldn't let him in. She came to the window and was cursing him and making the sign of the cross." Briony says her brother at one stage managed to get inside the house and see Kylie. "But he said that Virginnia spoke for her the whole time. My brother said, 'I want to hear it from Kylie', and so Kylie just repeated everything that Virginnia had said." I had expected Briony to express sympathy for her sister, but instead she says, quite out of the blue, "It's not the first time she's done this, you know." Kylie had gone to other families before. "She has embedded herself in other people's lives. Once she attached herself to someone in another church and didn't want to be associated with her family. But then they told her to go home to Nathan." Briony, who works in Sydney for a government child-protection agency, admits that what she has seen of the group worries her. "Once I went on a supervised visit, to accompany Liam. We met Kylie at a park in Richmond, but it wasn't just Kylie - there were other members of the group there, too, guarding her all the time. One woman was never further than an arm's length away, following her every move." Indeed, the only time I am able to speak to Kylie is when a female group member calls on her behalf. "I have Kylie here," the woman says. "She wants to say something." Kylie then reads a statement saying she has left an "unhappy and dysfunctional marriage" completely of her "own accord ... The people who have supported and cared for me for the past three years are my very dear friends and are not, and never have been, a cult."

Even Briony agrees that "it's not as simple as Kylie being kidnapped ... There's more to it than that. Kylie bears more responsibility than Nathan wants to admit." Kylie would seem to have lost a lot - her husband, her son, her job - but so has Virginnia Donges. "It's incredible, what has happened," says Martin Doran, Virginnia Donges's brother. Doran lives with his family in Springwood, not far from his sister. He says he was close to Virginnia, but that Kylie "broke our family apart. Now when Virginnia sees me, she crosses the street." He claims that Donges has begun to revere Kylie, whom she regards as "some kind of prophet. At one stage Virginnia said that, 'Everything would be revealed to the world in 2010.' Everyone round here thought there would be mass suicide." Fearing the worst, Doran and Donges's son, Brad, went to Springwood police, but they told them there was nothing they could do. "Virginnia doesn't do anything now for a job," explains Doran. "They got Kylie psychologically assessed, and now she is disabled in the eyes of the government and Virginnia is her carer." Doran says that Donges was "never the smartest tool in the shed", but that she was incredibly generous and "lived for her family". Then, in 1997, something happened that changed her forever. One day after heavy rains, her two middle sons, Grant, then aged seven, and Brad, 12, went for a walk in the bush behind their home. At one point they stopped to sit on a rock ledge, which broke under their weight. Brad was thrown clear, but Grant got trapped underneath. Brad tried to help but was unable to move the rock. Grant "looked at his brother, said, 'I'm dying', closed his eyes and went to be with the Lord", Virginnia Donges told a local newspaper.

"Virginnia had a breakdown after that," says Doran. "She slowly turned her back on everyone that mattered to her. We knew there was something wrong when her daughter, Karina, had a baby and Virginnia never went to visit her in hospital." Nathan paints a picture of Virginnia inducting Kylie into the group at the moment she was most vulnerable. But Doran says it was the other way around. "Everyone has a weakness, and Kylie has picked it." Some believe Kylie is not as ill as she appears. "I saw Kylie for a while, just after she left Nathan," a former member of the group says. "She'd go into this trance and then her voice would change and she would act like a scared little girl. At first it freaked me out, but then towards the end I kind of thought that she was having the other guys on." It also became apparent that many of the stories Kylie recounted were borrowed from He Came to Set the Captives Free, a controversial 1986 book written by an American doctor who claimed to have helped people escape the occult. Briony still regards Kylie as being "unwell" and in need of treatment. "I think that she and Virginnia are in a folie à deux, a kind of mutual delusion where they both feed off each other in some sick way."

In an emailed statement, Donges says she has been "supporting and caring for Kylie" through a difficult time. "I am in no way, shape or form controlled, manipulated or influenced by Kylie, nor have I ever been. I have no ill intent towards Kylie. I am not and have never been in a cult." Briony last spoke to Kylie a year ago. "It was about the fact she wasn't seeing Liam. I said, 'Liam is profoundly traumatised.' Even then I could hear Virginnia in the background, whispering to her. At the end of the conversation, I was so bitter about [it] that I hung up and erased her number." Nathan now lives with his parents on a two-hectare property at Oakville, near Windsor. The home he owned with Kylie is being rented. "We stayed there for a while but, after Kylie left, it was just so morose," he explains. I ask if he still wants to be with Kylie, and whether this may have clouded his perception of events. 'Well," he says, "I do find it hard to let go ..." Much of his time is spent lobbying government about the dangers of what he calls "psychological abuse". He has also petitioned the NSW Health Care Complaints Commission about the Blue Mountains psychiatrist who confirmed Donges's diagnosis of Kylie's DID. (The complaint was dismissed.) He has had more success with John Darnell, who was last year required by the ACT Health Services Commissioner to remove the Ministering to Dissociation Course Manual from the Shepherd's Heart website. Darnell has also posted a disclaimer, making it clear that his advice is "not intended to be a substitute for qualified professional opinion". All mentions of the word "dissociation" have since been replaced by the phrase "spiritual dissociation". Meanwhile, Nathan keeps busy. Last year he appeared in a local production of Dial M for Murder. ("Great fun.") He also took part in a re-enactment of Governor Lachlan Macquarie's visit to the Hawkesbury in 1810.

And every year, on Kylie's birthday, he drops off a card and flowers. Like Good Weekend on Facebook to get regular updates on upcoming stories and events www.facebook.com/GoodWeekendMagazine