An exhausted 21-year-old Mormon, Gary, is on his way back to Lancaster from a two-year proselytising mission in Utah, and he summons the energy to try to rescue one more soul on the plane. At home, the boy's ailing mother has been forced to leave the house for the first time in two years; his brother, father and sister are all carrying shameful secrets; and they're all hoping that his return will put everything right.

This is the third novel from 30-year-old Jenn Ashworth, who grew up a Latter-day Saint in Preston, and it takes place during a single day, with each family member taking it in turns to narrate. While the plot gets increasingly outlandish as it hurtles towards its climax, many of the book's best moments describe quiet, humdrum events. Gary's inner monologue as he patiently turns a conversation with a salesman towards God, trying to ignore his own stammer, is a juicy glimpse behind the bland facade of evangelism. "I count, and breathe, and do my anxiety exercises. He is my brother. He is my brother. Brother. My brother."

The portrait of religion that emerges is nuanced. On the one hand, we get 14-year-old Jeannie being taught that her chastity is like icing on a cupcake  once one boy has licked it off, no one else is going to want the cake itself  with terrible consequences.

Pauline, who has been incontinent since Jeannie's birth, avoids medical help and places her trust in God, and Gary struggles with the expectations placed on him as a model Mormon. On the other hand, the close-knit community provides solace and there's a glimmer of authorial approval in Jeannie's observation that her immodest classmates "think they don't have anything to offer the world but their skin, their hair and their eyes".

Ashworth's style is matter of fact and full of detail: the pattern of a sofa, the shape of a meal tray, the cracks on a path. She's also unafraid of ugliness. Every bodily fluid imaginable makes a cameo at one point or another, and desire, when it appears, is almost always shameful, deluded, suppressed, cruel or just plain wrong. This often makes for grim reading, but Ashworth appears less interested in spreading a simple, comforting message than in uncovering the messy complexities of people, families and faith.

In September 2011 Stagecoach Buses complained to the Mormon church about harassment of passengers by missionaries on the 2A from Lancaster to Morecambe. The Church of Jesus Christ of Latter-Day Saints, however, had nothing but praise for its 140 "persistent and courageous" young evangelists. Jenn Ashworth's third novel is a tragicomic tale of Lancastrian Mormons. She knows what she's talking about: she grew up in a Mormon household, abandoning the faith in her teens.

Mormonism, with the "aprons and the mirrors, the veils and hats and handshakes and chanting", is a comic writer's dream. But The Friday Gospels, written in a medley of five first-person voices, is warmly and sympathetically attuned to its characters' inner worlds. Each is hampered in some way by the bizarre ideology that twists the Leeke family out of true: wheelchair-using mum Pauline is only the most obviously disabled. Gary, returning from his mission in Utah, has converted nobody in two years; Julian is an oddball apostate; Jeannie is a sad child at the mercy of whoever elects to use her; Martin is a dog of a man, aiming to dump wife and family. It's a narrative of delusion, desertion and what the Bible calls "kicking against the pricks".

David Ebershoff's The 19th Wife and the TV series Big Love reflect modern fascination with the Mormon polygamous principle. But the Church of Latter-Day Saints disavowed polygamy in 1904, and Ashworth's characters are banal, straight-laced, monogamous folk who also happen to be Mormons. Their creed invokes so many prohibitions that forbidden fruit is everywhere: hence the education and supervision of the young is of crucial importance. In a sad-funny early scene, Jeannie attends a class in which the girls are allocated fairycakes topped by "white icing and glossy glacé cherries"; the boys are invited to "get your tongue out and lick [the cherry] off". Jeannie has to offer her partly eaten cake to another boy, who recoils: "He's had his gob all over it. It's disgusting." The message: keep your cherry undefiled for your husband. Women's wombs are the portal between heaven and earth, letting in souls from the realm of pre-existence. There are no second chances.

It's the week of Gary's return from Utah. Expectation burdens the novice missionary. Each Leeke family member is plunging into his or her own version of apostasy  and everyone hopes the missionary will clear up the mess. Martin the dad is a classic Ashworth character. His deepest passion is reserved for Bovril, "the bitch at my side". The choke chain with which he restrains the dog ("gentle reminder  it doesn't hurt her") represents the universal condition in The Friday Gospels. Everyone is choking, gagging, dragged along by a hierarchical patriarchy that polices its members.

Martin's emotional illiteracy is the handmaiden of delusion. Like Annie in Ashworth's very funny debut novel, A Kind of Intimacy, the paterfamilias doesn't do empathy; he hasn't a clue how people tick. He proposes to elope with young Nina, an unsuspecting dog-lover, deserting his sick wife. A literally shattering comic penalty is exacted from Martin. The fivefold plot surges towards mayhem and meltdown, cresting in a spree of comic excess. Any one character's problems would have been enough to ensure a saturnalian ending, but as the long-deferred climax comes, everything explodes  a greenhouse bursts into smithereens; Dad's erotic object becomes Mum's helpmeet; a black sheep finds a glorious mission. The plot is madly over determined. But in the end, who cares? Justice is done.

Ashworth's language is never less than inventive and exuberant, and her observations are minute. Omission is sensitively used to express Jeannie's speechless, flinching shock at what has been done to her: "He. Then he. After a while he. Then I. When he." Pauline's paragraphless flood of thought carries her from panic to panic.

Pauline, the pious but helpless matriarch, is perhaps the novel's greatest success. She's fundamentalist through and through, fearing "Satan's grip on technology  [the internet] a sewer pipe in your living room", bullying her daughter, alienating her husband and son, rancorously humiliated by incontinence. But the reader feels for her; the first-person narrative exposes her private suffering and struggle to our pity. In Ashworth's final dispensation of comic penalties and compensations, Pauline finds release both from delusion and illness  which come to much the same thing.

 Stevie Davies's Into Suez is published by Parthian.

In September 2011 Stagecoach Buses complained to the Mormon church about harassment of passengers by missionaries on the 2A from Lancaster to Morecambe. The Church of Jesus Christ of Latter-Day Saints, however, had nothing but praise for its 140 "persistent and courageous" young evangelists. Jenn Ashworth's third novel is a tragicomic tale of Lancastrian Mormons. She knows what she's talking about: she grew up in a Mormon household, abandoning the faith in her teens.

Mormonism, with the "aprons and the mirrors, the veils and hats and handshakes and chanting", is a comic writer's dream. But The Friday Gospels, written in a medley of five first-person voices, is warmly and sympathetically attuned to its characters' inner worlds. Each is hampered in some way by the bizarre ideology that twists the Leeke family out of true: wheelchair-using mum Pauline is only the most obviously disabled. Gary, returning from his mission in Utah, has converted nobody in two years; Julian is an oddball apostate; Jeannie is a sad child at the mercy of whoever elects to use her; Martin is a dog of a man, aiming to dump wife and family. It's a narrative of delusion, desertion and what the Bible calls "kicking against the pricks".

David Ebershoff's The 19th Wife and the TV series Big Love reflect modern fascination with the Mormon polygamous principle. But the Church of Latter-Day Saints disavowed polygamy in 1904, and Ashworth's characters are banal, straight-laced, monogamous folk who also happen to be Mormons. Their creed invokes so many prohibitions that forbidden fruit is everywhere: hence the education and supervision of the young is of crucial importance. In a sad-funny early scene, Jeannie attends a class in which the girls are allocated fairycakes topped by "white icing and glossy glacé cherries"; the boys are invited to "get your tongue out and lick [the cherry] off". Jeannie has to offer her partly eaten cake to another boy, who recoils: "He's had his gob all over it. It's disgusting." The message: keep your cherry undefiled for your husband. Women's wombs are the portal between heaven and earth, letting in souls from the realm of pre-existence. There are no second chances.

It's the week of Gary's return from Utah. Expectation burdens the novice missionary. Each Leeke family member is plunging into his or her own version of apostasy  and everyone hopes the missionary will clear up the mess. Martin the dad is a classic Ashworth character. His deepest passion is reserved for Bovril, "the bitch at my side". The choke chain with which he restrains the dog ("gentle reminder  it doesn't hurt her") represents the universal condition in The Friday Gospels. Everyone is choking, gagging, dragged along by a hierarchical patriarchy that polices its members.

Martin's emotional illiteracy is the handmaiden of delusion. Like Annie in Ashworth's very funny debut novel, A Kind of Intimacy, the paterfamilias doesn't do empathy; he hasn't a clue how people tick. He proposes to elope with young Nina, an unsuspecting dog-lover, deserting his sick wife. A literally shattering comic penalty is exacted from Martin. The fivefold plot surges towards mayhem and meltdown, cresting in a spree of comic excess. Any one character's problems would have been enough to ensure a saturnalian ending, but as the long-deferred climax comes, everything explodes  a greenhouse bursts into smithereens; Dad's erotic object becomes Mum's helpmeet; a black sheep finds a glorious mission. The plot is madly over determined. But in the end, who cares? Justice is done.

Ashworth's language is never less than inventive and exuberant, and her observations are minute. Omission is sensitively used to express Jeannie's speechless, flinching shock at what has been done to her: "He. Then he. After a while he. Then I. When he." Pauline's paragraphless flood of thought carries her from panic to panic.

Pauline, the pious but helpless matriarch, is perhaps the novel's greatest success. She's fundamentalist through and through, fearing "Satan's grip on technology  [the internet] a sewer pipe in your living room", bullying her daughter, alienating her husband and son, rancorously humiliated by incontinence. But the reader feels for her; the first-person narrative exposes her private suffering and struggle to our pity. In Ashworth's final dispensation of comic penalties and compensations, Pauline finds release both from delusion and illness  which come to much the same thing.

 Stevie Davies's Into Suez is published by Parthian.