By the time he died, in 2005, he had become a prophet honoured in his own country and in his lifetime, an unusual fate for a Scot.

Like countless others, I discovered him from the paperbacks that began appearing then. There was The Cone Gatherers, his haunting forest novel of the Second World War, still his best-known book; but then Fergus Lamont, Just Duffy, Dust on the Paw and many more. A fine novelist has an atmosphere, a saturated colour, all his or her own, and Jenkins has it. It's ultimately indescribable, something to be seen or tasted, but it has something to do with moral intelligence, social conscience, an acute understanding of life's absurdity, and deep human sympathy.

John Jenkins (as he was known outside writing) had been brought up in a tough Lanarkshire mining village in deep poverty; he became a schoolteacher, a wartime conscientious objector working in Argyll's forests and, for a while, a militant socialist. Yet after the collapse of his party, the Independent Labour Party, he described himself as a moralist, and it is the moral cutting edge of his writing that makes him a lot more directly challenging than most contemporary novelists, even if they are slicker and subtler in style. At his finest he is the Scottish Ibsen, or even Chekhov, in the intensity and dark plunge of his thinking. My favourite for a long time was an historical novel, The awakening of George Darroch, about the least likely of subjects, the struggle of a Church of Scotland minister over whether to abandon his living when the Kirk split over its relationship with the early Victorian state. Jenkins took this very important but also musty-smelling issue as the background for a book about conscience, frailty and oppression that could help anyone today understand life in communist Eastern Europe, or indeed the agonising choices currently faced by Islamists in Pakistan and Afghanistan – a part of the world Jenkins knew well.

Then I came across The Changeling. It usurped even George Darroch, which it much resembles. The setting is nearer to us in time but yet a world away. It tells the story of a fat, sentimental, not very successful schoolteacher who believes in human goodness, and who tries to reclaim, or save, a bright boy from the Glasgow slums by taking him on his family holiday.

Not so long ago, middle-class people did sometimes do things like that. The more Christian atmosphere of Scotland in the mid-20th century, and the equally idealistic socialism of the time, could provoke people to reach out, as they don't today, after another half-century of rising incomes and waning faith – religious and political. Even in 1956-7, roughly the time of the novel's setting, most people see the teacher's action as naive and possibly hypocritical (is he trying to attract attention to get a promotion?). But it was less outlandish then.

JENKINS WAS INTERESTED IN NAIVE

people, including those trying to do good in a cold, materialist word. In The Changeling (republished by Canongate this week) the teacher, Charlie Forbes, is both risible and impressive. The story depends upon him being, like most of us, a curdled mix of vanity, ambition, selfishness, sentimentality – and altruism. We are frequently reminded of his clumsy absurdity; on a bicycle with buckled wheels, "in mauve corduroy shorts, leather moccasins, and white open-necked shirt that revealed the hairiness of his broad chest, he sang as he zigzagged in the sunshine". He responds to the beauty of the Argyllshire landscape with unrestrained romanticism, a mix of half-digested history and childish romance. He is a humbug and a bore – as one of the other teachers tells him to his face, "smug and phoney". And indeed he is almost completely impossible.

But only "almost". Though most of those around him reckon that he is a hypocrite, Forbes is more interesting than that. He has had Quixotic moments before. Once he invited a labourer to join a family picnic – almost casually we learn that eight years on, the bitter and bereaved man recalls the tea "still sweet in his mooth". Random acts of goodness reverberate. He knows it, even if hardly anyone else seems to remember. In his galumphing, ridiculous way, Forbes retains a fundamental optimism about humanity without which we are lost souls.

So he is genuinely moved by the sharp brightness of the slum boy, Tom Curdie. Unlike the rest of the school staff, his wife, children and mother-in-law, all more cynical, he doesn't see Curdie's half-smile as insolence, but as rather brave. His scheme for Curdie has a trickle of sly ambition about it, but even Forbes cannot work out how much. At one level, the other teachers, who are brutal, coarse and unethical, are right and he is wrong. Curdie is indeed a "practised liar and a thief". He is deep and devious. Living with an alcoholic mother and a crippled stepfather in a tenement with his brother and sister, he is brutalised, veiled, armoured. In a few strokes, Jenkins draws his life of industrial poverty as deftly as Dickens, and without a flicker of sentiment. There are poor people. And partly because they are poor, they turn out to be nasty, too. Here, there is nothing redeeming about poverty. Donaldson's Court, the slum where the Curdies live, is a place which stands for the worst in modern life. The others are right to fear and shun it.

Yet, at a more important level, Forbes is right. Curdie steals for money to help his friends, but also he steals to keep himself from going soft. The frightened, beaten-down, beautiful human being spotted by the teacher is there, hidden behind his circumstances and intelligence. For him, stealing is something protective, a palisade that will shore up the distance between him and the Forbeses. It is self-definition of a pessimistic kind. It keeps out feelings that might weaken him in the vicious fight for survival that is slum life. If he lets himself believe he belongs in the comfortable middle-class Forbes family, with their good food, indoor lavatories and country holidays, then when he returns to reality, life will be intolerable.

For Curdie is not being offered adoption. He is not being "saved". He is being given a tantalising glimpse of a brighter, more beautiful world, before it is snatched back and he is returned to a life that stinks of piss and booze and failure. This better world is embodied in the fictional Argyllshire village of Towellan, an apparent Arcadia which represents for the Forbes family their annual paradise. Like the Borders countryside in Hugh MacDiarmid's poetry, or Orkney in the writing of Edwin Muir, it stands for a dream of uncorrupted, pre-industrial human life, to be set against the worst of Scottish mid-century slums. To the Forbeses, "Towellan" means family harmony, unforced laughter and shared experiences as banal as favourite walks or unsuccessful fishing trips – a seam of joy to be hoarded and savoured for the rest of the year. So to take Curdie with him, Forbes believes, is not a small thing. It may somehow inoculate him – with optimism, or vital human spirit. It will at least show him another way of living. That this is both well-meaning and obtuse, even cruel, is something that hard, bright Curdie instinctively grasps, but his teacher, brimming with naivety, is simply too stupid to understand. What is he about? Does he think he can save someone with a fortnight in a cottage?

So this is a moral battle, fought out by ordinary people, who rarely understand one another yet for whom time is short. Forbes, apparently risking nothing worse than a little gentle ridicule, finds himself risking and losing much more than that – in effect, his own human essence, for he becomes monstrous by the end of the story. Curdie, who thinks he is well protected from love and pity, is physically destroyed because Forbes has done his (wholly well-meaning) work and shown the child the full horror of returning to his own background, while knowing that he cannot stay in paradise either: he literally has nowhere left to go. His instinctive sense that he had to protect himself, build a wall, is shown to be bitterly accurate. Meanwhile, the more hard-bitten characters around these two have risked nothing and lose nothing, yet they are shown without sympathy, as lost souls clothed in modern banality. Only Forbes's daughter, Gillian, who starts by hating Curdie and wants to destroy him as a spy and sneak, to get him out of the family, is transformed. If there is a scrap of hope in the book, it is her journey from selfish hard-heartedness to deep empathy, of a kind that might in future be less clumsy and self-regarding than her father's.

As the action advances, barely a word or phrase is wasted. Everything is charged with moral meaning, from the diseased rabbits deliberately infected with myxomatosis to improve farmers' profits, to the Americanisms of Forbes's son, who calls his father "Pop". In the apparently timeless Argyllshire of the late 1950s – Jenkins's chosen home landscape for much of his life – the world's politics is not so far away. The Holy Loch will very soon be visited by the first US navy Polaris submarines and protestors will be marching along the roads where Forbes on his wobbly bicycle had teetered, dreaming. In the 1950s, package holidays abroad would soon dilute the old raucous Glaswegian rituals of going "doon the watter". Pop music would kill off the open-air singing competition shown in the book – indeed, it apparently already is dead. The teachers belting and abusing slum children are themselves on the way out. So too is the old culture of Scotland generally, so sentimentally admired by Forbes. Anyone who still thinks that Britain in the 1950s was a reassuring and stable society would be well advised to read this novel attentively.

This being Jenkins at his best, all the characters are human, fallible and at times ridiculous, from the middle-class mother-in-law to the drunken slum parents. Except for Curdie and Gillian, they appear at first as actors in a comedy, not a tragedy. There is no getting away from the fact that this is a bleak little book, for all the sunlight and summer laughter. It makes me cry, and unsettles me badly, and yet I pick it up again, and press it on friends. It has settled itself somewhere inside my mind, and stayed. What more can you say?

The Changeling, by Robin Jenkins, is published by Canongate, priced £6.99.