Nineteenth-century Europeans loved a sentimentalist, and Therese of Lisieux, dying of tuberculosis in a nunnery at 24 after having written a popular autobiography, suited the times. But why is the Catholic church preparing new honours for her, asks Richard North, who argues that candidates for public canonisation, from Che Guevara to Bob Marley, have much in common.

On 30 September 1897, an obscure young nun died. Therese Martin was a Carmelite, living in her strictly enclosed order in Lisieux, a provincial city in northern France. She was 24 and had succumbed to tuberculosis. That might have been that - a frequent occurence among people of that time, a family sadness, but only a small blow to a community devoted in any case to preparing people for the afterlife. But Therese was something else. She had always intened to be a saint. Her image was soon in a thousand breast pockets in the trenches. It sustained sweaty missionaries all over the tropics. In 1925 she was canonised. A hundred years on, she will be made a Doctor of the Church tomorrow, in St Peter's. Henceforth, her dicta are to be considered of a quality with those of St Thomas Aquinas and the other builders of Catholic doctrine.

How come? Therese became famous for a rather sickly faith: she called herself a "little flower of Jesus", and thought of herself as a plaything of Christ in addition to the normal nun's thought that she was a bride of Christ. Like an anorexic, she sometimes seems someone resisting adulthood. Her self-abnegation at times seems - probably wrongly - like an insult to the aspirations of the rest of her gender. Still, if she was sentimental, it was a sentimental age. If we struggle to see her merit, her generation might have struggled to see that of the late Princess of Wales, who has become so admired in death.

We cannot afford to be too superior. A hundred years later our appetite for such dead heroes does not diminish with the waning of conventional faith. And while we are perhaps more cynical than our forebears, we seem to be at least as - perhaps more - credulous. The young like a romantic hero, perhaps to keep alive the belief that however humdrum their own lives, there remain people who combine talent and drama. The old, no less than previous generations, are in need of gaining consolation from the idea that there are people who personify goodness.

So "the people" will go on making - or rather, proposing - saints. Mother Teresa of Calcutta may have been an erratic administrator, as is rumoured, but unless something much worse turns up about her, she is likely to remain admired. Hindus in Calcutta are already venerating her (along with Princess Diana) and in time the Church will presumably decide whether she can be endorsed as truly holy.

The non-religious also make saints. Though they have fewer rules to go by and sexual morality and personal denial are much less of an issue, the same types emerge. Che Guevara died thirty years ago, and for twenty years he was a vivid icon of revolutionary and military fervour. The comparison is necessarily imperfect, but for those students who put up the poster with his beret-ed head, Che was a sort of Joan of Arc, the 15th century French freedom fighter and visionary who had to wait 500 years for canonisation. The image of Che on a T-shirt is, by the way, strikingly similar to medieval portrayals of Christ's face imprinted on the white cloth which Veronica passed to him for comfort on the way to Calvary, and which was imprinted with his countenance. Not, of course, that Christ advocated violent revolution.

Che was a guerilla, and executed by his captors: this makes him, like Joan, a candidate for the label of "martyr". He had often prefigured such an event. This isn't a perequisite for sainthood, but it certainly helps. Paying the ultimate price for one's beliefs is particularly potent as an ideal in the romantic and moral hero. It is, after all, altruism taken as far as it can go, rather as a suicide pact is the ultimate test of lovers. Saints are above all extremists.

Martial heroes do not make obvious spiritual heroes, and yet their deaths - like Che's - can have elements of martyrdom about them. And they can be romantic, too. Nelson acquired something like saintliness even when he was alive, and his death provoked a violent upwelling of popular feeling. Stephen Daniels, the cultural critic, sees him as an important popular, as opposed to state, hero. He was a hero at arms, but like Joan of Arc manages to mean far more than, say, Churchill. He was Byronic, a lover, a self-made man, a man driven to heroism out of an attractive weakness of spirit.

For a generation that has not known war, is not ideological but is not formally religious either, it is hardly surprising that Che's image on adolescent bedrooms has been replaced by that of Bob Marley. I fell under the reggae star's spell when I interviewed him, and happily propose that he was some sort of saint. His womanising and his laddishness might have counted against him, and he may have had more serious faults I know nothing of. Perhaps the kids like the spliff which is forever in his hand or lips. There is something more to it than that, though. His musical outpourings would bear comparison with St Francis' mystical but earthy Canticles, and were strikingly resonant with the Psalmody he must have heard in Jamaica as a child. It was song-making of profound seriousness and great joy. That would be enough to make him worthy of attention. But there is something, too, in his apparent lack of self-regard. One guesses that charisma (which he personified) equals stardom in inverse proportion to vanity. I never believed that he took Rastafarianism all that seriously, except as a mild and decent myth and ritual which conferred dignity on a people in serious need of some. Still, he may also strike people as having a sort of devoutness about him. His songs of peace seem far less fatuous than poor John Lennon's.

Courage, martyrdom, sanctity, youth: no one of these things is necessary or sufficient to sainthood. But candidates need to exemplify two or three among them. And it helps, too, to have been flawed, profligate or reckless at some point. Where else lies proof of renunciation, or redemption? Leonard Cheshire, a war hero and man about town who became a devout charity worker, might make a candidate for sanctity. He used to argue that physical disability - his special care, but psychological disability must be the same - is a mark from God. It provides unique opportunities for insight in the victim and sympathy in the onlooker.

And how we fail the unfortunate, and then venerate them. Thus, Marilyn Monroe's death is iconographic because it is supposed that men, bewitched by her beauty and untempered by self-discipline, played on her flawed nature and put it, finally, into a flat spin.

The public always likes its sympathies to be tinged with shock. Wilde, Hendrix, Morrisson, Moon prove it. James Dean's death in a smashed Porsche matters because it was appropriate to our image of him that he burned out in the fast lane of life. St Francis of Assisi had been flashy: a follower of fashion to please a Versace. St Teresa of Avila remarked how violently she desired the things she wanted: like Freddie Mercury (whose death inspired admiration out of proportion even to his success in life), she could say: "I want it all and I want it now".

Seen against these criteria, Therese was burdened with or achieved more than enough of the requirements of sainthood. She put her signature on a sort of spirituality. At 16, and as a nervy, spoiled brat, a creature of a comfortable bourgeois home, she had bullied the church authorities into allowing her to incarcerate herself in the local convent. Refusing to ask for medication, she died a heroic and largely voluntary medieval death. But she was a determined youngster, set on sainthood. She had been prevailed upon to write an autobiography and it is not going too far to say that it was assiduously promoted by a formidable gang of her fellow religious.

But The Story of a Soul, as it was called in English, belies everything a cynic might say of it. One of her most recent biographers, Monica Furlong, points rightly to the steel and the good sense which it displays. Therese, on this reading, might have outgrown the sugariness into which the chauvinist culture of her day had indoctrinated her. She might, in short, have grown more like that grander figure, Teresa of Avila who herself was a hysteric in early years and achieved greatness only after her 40s.

And even as it stands, Therese's legacy is serious. A youngish nun told me in Lisieux this summer that Therese's devotion to the family, and her use of the human family as a simile for the heavenly family, is all the more poignant now that we have so damaged its structures. But a doctor of the church? For at least one senior Catholic lay figure, this seems a dumbing down too far. But then, the Catholic church endures above all because along with its authority, it never eschews the vulgar, the simplistic and the dissident if it believes these things have caught the attention of enough people of goodwill. It has, after all, the image of Jesus - victim, virgin and visionary - to uphold.