FICTION : A RECENT MARTYR by Valerie Martin (Houghton Mifflin: $16.95; 204 pp.).

Sex in “A Recent Martyr” is riveting, mature and believable. Theology is not. In a book that supposes to address the question of an individual’s relationship to God, this matters.

Emma is having a torrid affair with Pascal and then they each meet Claire. Claire is a virgin, an aspiring nun and the possessor of the ambition to one day be a saint.

Pascal is obsessed by Claire. Knowing Claire makes Emma give up her sexy encounters with Pascal. There is a plague in the city--New Orleans in this case--and in the end, Claire dies as the victim of a rape and murder. The story is not the best part of Valerie Martin’s new book.

Emma gives up Pascal, we are supposed to believe, because of Claire’s example. Common sense would have told her to give up a lover with a loose knife and an affinity for pain. But this is not a book about common sense. It is a book about passion, and Martin writes about it very, very well.

But it strikes one that this is a book for Roman Catholic girls who lost their innocence, grew up to be feminists, entered the world but who never lost their fondness for or awe of nuns. It is as if the sex grew up but the theology did not.

The God of this book is a God who punishes the innocent, is not impartial and universal and who works in ways so inscrutable as to be unbelievable and almost absent. Unfortunately, good writing and steamy sex do not an exploration into the magnificence of Deity make.

The book is dedicated to “M.A.,” whom I presume to be author Margaret Atwood, a mentor of Martin’s. “A Recent Martyr” is an interesting two hours’ worth of reading. It would not, however, be unfair to say that you don’t have to have a Roman Catholic girlhood to appreciate this book--but it would help.