The trials of Mary endure.

She stands awkwardly or sits uncomfortably in a chair in a rudimentary room as the greatest story ever told is prised from her in an operation that is both painful and intimate. Mary, mother of Jesus, is to be enshrined as Mary, mother of God. There is a deep unease as she faces the chroniclers of a story that will launch Christianity and spawn thousands of cults and sects over the next 2000 years.

Colm Toibin's remarkable and beautifully rendered novella places Mary in front of two writers, presumably working on the gospels. Given the clues, one is almost certainly John, the disciple and writer of the most deeply spiritual of those testaments. But Toibin, artfully, leaves their identities spare and nameless. This is Mary's story and their persistent questions only serve to reflect the endless fascination over a faith that took root in the Galilee of a misty past and reached its most significant moment with the death and resurrection of Jesus Christ.

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Mary testifies to the certainty that her son lived and died but remains cautious, even gently confused about much else. How could she be anything else? The brilliance of Toibin's portrait of Mary is that he makes her human in a story that is supernatural. He is also too sensitive, too sharp to be involved in any crude debunking of Christianity or blunt certainty about matters that defy human understanding.

However, this is a testament and it includes testimony. Thus Mary insists Lazarus was dead and buried before the intervention of her son, the cripple walked when told to take up his bed, and at least one of the jars at Cana was water before the wedding celebrated an infusion of wine. She is also perceptive on both her son and his followers. The disciples were misfits, she says gently. Her son, too, grew away from her and towards an inevitable early death.

Mary, too, is dying, and reconciled to this fate. But the world cannot let her go. It never will. The insistent interrogation of the scribblers writing their testament to Jesus and his eternal significance irritate and frustrate her. "Before that final rest comes this long awakening," says Mary, facing the life and death of her son yet again, this time in a draining question-and-answer session lasting days, even weeks.

Toibin is imprecise about time but wonderfully acute on Mary, an old woman both scarred and formed by the past. Millions believe Mary was central to a supernatural event: the birth of God in human form. Everyone must believe she was witness to an unnatural event, namely that of a mother watching her son die.

This is Toibin's testament of Mary on the moment when Jesus, bloodied and awfully wounded, stumbles under the cross again on the road to Calvary: "He was the boy I had given birth to and he was more defenceless now than he had been then. And in those days after he was born, when I held him and watched him, my thoughts included the thought that I would now have someone to watch over me when I was dying, to look after my body when I had died.

"In those days if I had dreamed that I would see him bloody, and the crowd around filled with the zeal that he should be bloodied more, I would have cried out as I cried out that day and the cry would have come from a part of me that is the core of me. The rest of me is merely flesh and blood and bone."

This is anguish, the humanity of the story exposed. Toibin's work resonates with that awful grief of a mother, the guilt of Mary when she reveals she fled in fear when her son died, and then humbly accepted not fully understanding who she may have created and what has followed.

Toibin portrays Mary as a quietly holy woman, one who prays, who "does not need much" and who makes her judgments gently and without a sense of superiority. She emerges as a genuine figure of force, a credible witness to the most spectacular and baffling of circumstances. Mary, too, is fully human in both her strength and her weakness, and her inability to linger on the former while berating herself for the latter.

Toibin's story and characterisation is thus wholly authentic. His considerable gifts ensure that a credible Mary emerges from this novella. This creation of truth is praiseworthy. His triumph, though, is to ensure that the mystery remains.

Colm Toibin is at Dundee Literary Festival at Dalhousie Building on Monday at 7pm. Tickets £5, £3 concessions. or call 01382 386995