By Susan Howatch

Knopf. 386 pp. $21.95

In the Middle Ages, Good and Evil strode around onstage in morality plays that addressed the major concern of the times: Would one's soul rot in Hell or be lifted up to Heaven for a happy eternity?

Nowadays, for those who think of it at all, Hell is the New York subway after 10 p.m. and God is a great guy who didn't really mean the Ten Commandments.

The old and new versions of God meet in Susan Howatch's novel "Scandalous Risks," the fourth in a series examining the Church of England during this century. Which sounds very dull indeed.

It isn't because Howatch is a born writer of romances, the kind of book one wants to cuddle up with on a dark winter's day. But where most romances force the heroine to choose between Mr. Right and Mr. Wrong, Howatch's heroine must choose between the delectable Mr. Wrong and doing right.

Her heroine, Venetia Flaxton, is the daughter of the agnostic Lord Flaxton, an irascible peer who considers the Church of England one more club a man should join. Venetia, last of the family's large and socially successful litter, is awkward and out of step. Constantly quarreling with her father, spending her days sipping gin and listening to Elvis Presley, Venetia despairs of ever finding even so pitiful an escort as the one flaunted by her best friend. She may mock her friend's taste but is forced to admit that in her own life, "no limp, damp individual had presented himself for acquisition."

The problem with Venetia, according to the bishop of Starbridge's wife, is that she is not making the most of herself. Within days, Venetia has changed from horror to houri, a pre-Raphaelite sizzler with a parade of possible escorts.

But before this new social world can open up for Venetia, another world encloses her. She is lured into the arms of her darling Mr. Dean, Neville Aysgarth, dean of Starbridge Cathedral, her father's best friend and a man 35 years her senior. Aysgarth, disastrously and firmly married to a clinging, difficult woman, is able to embrace Venetia because he has also embraced the new morality, as expressed in a popular book written by a theologically liberal bishop, John Robinson. This modern man has written that anything done in the name of love is A-okay with God.

Bishop Robinson may not have meant to unleash the demons of desire with his startling and, not surprisingly, popular interpretation of the old saw "God is love," but that is what he has done. Venetia's employer, the bishop of Starbridge, is appalled at this interpretation of Christianity and lectures her on a sterner God, while Neville Aysgarth, who admires Robinson's book, entices her into the Age of Aquarius.

Aysgarth assures the happily acquiescent Venetia that they can fondle without fear because, as Robinson has written, the purity of their love will keep them chaste. Aysgarth is a complex man, so removed from the consequences of his acts that he has divided himself up like an archaeological dig: Neville One, Neville Two and now, in Venetia's arms, Neville Four. He does not consider himself responsible for the errors of these earlier Nevilles, creatures who have crawled away from him and gone off to die.

Howatch is at her best when dealing with conflict, bringing a passion and tension to her portrait of people facing moral dilemmas and trying to reach out to God. The theological core of her novel is lightened by her picture of the debs of the day, 1960s Bright Young Things whose wicked ways are as charming and innocent as the slang they speak in. It is this social setting that gives the book a fullness it might otherwise lack and makes Venetia's infatuation with Aysgarth seem plausible.

Although characters from Howatch's earlier novels slide in and out of the pages, you do not have to have read those books to enjoy this one. The story stands on its own and Howatch's fans will simply find an added pleasure in recognizing old friends as they pop in and out.

As good as Howatch is on writing of the conflict between man and God, when she attempts to bring everything together with a resolution, her style fails. Venetia's salvation has a blurred, artificial feel, like those cheap lithographs of people ascending to Heaven, toes pointed downward and faces indicating how very surprised they are by their sudden and unexpected elevation.

The reviewer is a writer and critic living in Maine.