A train slows its progress through the English countryside and stops outside a small town: "An elderly woman and a younger man were coming down the steps. The man carried a suitcase in one hand and held the arm of the woman with the other . . . They were a striking couple, tall and confident. Mother and son.

"They were closer now, on the platform, and a pattern of light through the gaps in the station roof streaked the son's face. It was a reddish colour, Book World OCCASION OF SIN. By Rachel Billington. (Summit Books. 315 pp. $14.95) with broad cheekbones, large nose, large eyes, low forehead and bristly fair hair cut short. It was the sort of very masculine face which did not appeal to Laura."

It was, she realizes later, a face she had at that moment fallen in love with. And thus begins the disintegration of Laura, beautiful wife of a brilliant barrister, mother of a son whose presence fills her with joy, a woman who can sit for hours in peaceful idleness, watching the light change in her living room. She is lulled by the daily objects because they are the small things that make her marriage; and for her, marriage is a state of grace.

Safe inside its bounds, she can be strong, accepting the love of her domineering husband, the admiration of friends, "able to help other people without becoming too upset, too involved."

But Laura's beliefs are careless things, a comfort she hasn't earned. Echoes of conviction, they serve her less well than the simple self-knowledge of her sister-in-law, who forgives her husband his endless infidelities because "I do love him . . . Besides, I like being married. I like having a man in bed. I like having a husband . . . even if he isn't a very good one."

Laura's marriage is based on ease, not love. Her Catholic childhood has left her with guilt at the thought of breaking her marriage vows, but it has not given her the strength to keep them. ". . . her love, her lust for Martin is a good old-fashioned sin. A mortal sin, a purely wrong, self-destructive act . . . She remembered childhood catechism lessons. 'Avoid the occasion of sin.' Agreeing to the occasion of sin was as bad as the sin itself. She must not see Martin again."

But of course she does. The obsession with love fills her as completely as duty had earlier. But less predictably. The calm, caring Laura is gone. In her place is a woman who slips out of her marriage in anger and irritation at her husband, who grieves when her son is taken away but does nothing to keep him. For Laura, as for so many people today, passion is not merely redemption, it is an excuse to do what she wants. Love, for Laura, is a discovery of herself, and she is unaware and unconcerned about the effects of her behavior on others. Even her adored Martin is not really known. What is important is not how love affects him, but how his love affects her.

Not 'til the end of the book, when passion spends itself, does Laura see people as they really are. Her husband is not self-sufficient and dominant, but a man who has drawn strength from her; Martin's mother is not a cruel woman but an old and lonely one. And Martin? Martin is merely a man "with a reddish face, tired and kindly. A man who is neither young nor old but nearer old. A man who has travelled the Atlantic to see her and has a small split in his anorak . . . Martin looked ordinary, like any other man. She had hit the end of the tunnel with a soft bump and found herself sitting in a nice cosy living-room."

Laura is no longer special, no longer protected by virtue. In abandoning her state of grace, she has lost the certainty that gave her life a center. There is a baby now, and possibly she and Martin will have a life together. Possibly not.

Her friend Nell thinks, "Poor Laura . . . nothing will ever be the same again. It's almost enough to make you believe in the apple and the Garden of Eden."

The pride and the paradox of lovers is their conviction not only that they are the first to love so deeply, but that, by their actions, they have become linked to the great lovers of the past who felt exactly the same thing.

Billington wants us to know that her lovers are unique, but that they are also a cliche', speaking over the chewed-over words of a thousand love affairs.

" 'Why are you crying?'

" 'You know.'

" 'Tell me.'

" 'Because I love you. And because I never thought it would be possible.'

" 'What?'

" 'To feel like this.'

"Martin cradles her proudly."

"Poetry is intensity and nothing intense lasts for long," E.B. White once wrote, criticizing a poem whose length outran its sense. Billington would say the same of passion and has shaped her book in a series of scenes that shift from London to New York to Italy to Ireland, each as narrowly focused and as intense as passion itself. What happens when intensity goes is for us to decide, but in her perceptive and well-written book the author not only returns us to an awareness of sin, but reminds us that though the state of grace can be easily lost, it can also be regained.