FASHION SELDOM admits of any studied explanation. Wavelike, trends rise, crest, fall and rise again seemingly without reason. Even fashionmongers, the people who devote themselves to charting these periodic cycles, make no pretense of perceiving a rational causality in it. Rather, they channel their energies into sniffing out the true trend-setter-or-reviver from among the herd of recherche hangers-on who persist in feeding off the piled carcasses of defunct trends.

Agnes of God, a new work by John Pielmeier, now playing for two weeks in Boston before relocating to Broadway, requires a fashionmonger's scrutiny. As a psycho-drama of the good old school, this three-person play is unmistakably out-of-date. But double-breasted jackets with padded shoulders, which went out decisively after the '40s, are currently enjoying a big vogue in the very haut-est of designer salons. So, perhaps, Agnes of God might take the theater world by storm and herald the revival of the psychodrama as a popular form.

Certainly the play boasts a gripping plot: it takes as its subject the true story of a young nun who became pregnant in her convent and whose baby was mysteriously strangled just after birth. But the script's forays into the realm of allegory place a heavy burden on its dramatic content. Agnes (Amanda Plummer), the nun on trial for the murder of her child, and her Mother Superior (Geraldine Page) together represent faith; Dr. Martha Livingstone (Lee Remick), the psychiatrist assigned by the court to the case, embodies reason. The problem with the play lies in its clumsy handling of so diametric a dramatic opposition Perhaps because Pielmeier himself was born and raised a devout Catholic, his writing openly favors faith over reason.

First, to cast psychoanalysis as the soldier of science in the battle between mysticism and rationalism seems to handicap the side of reason. While analysis is currently in retreat from more purely scientific methods of treating mental disorders, notably psychopharmacology, religion remains the purest form of faith. The combatants in this battle begin on inherently unequal terms, then, because Catholicism has every right to carry the standard of mysticism, while psychoanalysis can make at best a feeble claim to the banner of nationalism.

BUT PIELMEIER has taken a stand against reason in a more immediately disturbing manner: his Dr. Livingstone cannot compare with the two nuns. Cold, neurotic, self-righteous, she has none of the appeal possessed by the waiflike Agnes and the buxom, comfortable Mother Miriam. It seems no accident that this play, unlike most concerned with the conflict between faith and reason, plumps squarely for faith and leaves its audience with a neatly-wrapped package, for faith and leaves its audience with a neatly-wrapped package, satisfied that a decision has been reached and a conclusion obtained.

Despite the incomplete sketching of Dr. Livingstone's character, she has the honor of delivering the play's opening speech--which, laden with dreadfully kitschy symbolism, has the effect of striking a desperately false note. Such notes recur, though not at all frequently, and most are sounded by Dr. Livingstone. Her chain-smoking, which she explains as an obsession taken on when her mother died to replace her former obsession with that daunting figure, is unattractive, intrusive and psychologically simple-minded.

In fact, were most of the acting not so fine, the play would be crippled by its overt bias toward faith over reason. Because Geraldine Page and Amanda Plummer perform so magnificently, Pielmeler's faulty script sails along smoothly.

Page, a grande dame of the stage, shapes a Mother Superior who is at once completely convincing and utterly likeable. Pielmeler's firsthand experience of parochial schools obviously has served him well in one respect: he draws his nuns, if not his psychiatrists, clearly and beautifully. Intelligent, knowledgeable about the world outside the convent (she was married for over 20 years and bore two children). Mother Miriam holds to her faith more fiercely than a sheltered lifelong nun might. She believe she wants to believe not because she can conceive of no alternative.

Since she understands life in the secular realm, Mother Miriam is uneasy about allowing the forces of the outside world entry into the sanctum of her faith. Having consciously created for herself the stronghold of religion, she has pulled up the drawbridge and refuses adamantly to let it down again.

Yet, a thoroughly wise woman, the Mother Superior remains aware of her self-imposed blinders and seeks an alternative justification for her faith. She wants a miracle, tangible proof that her choice was not wrong, in Agnes, Mother Miriam believes she has found that miracle. Agnes has at ethereally lovely singing voice--an apparent gift from the angels. She remains touchingly childlike, innocent of the facts of life, of her own body, of people. She is, Mother Miriam claims, a human being whose innate ties to God have not been sundered by the exigencies of life and fate.

But Agnes is not merely an innocent; she is also a 21-year-old woman who has given birth and who suffers from a cornucopia of mental imbalances Mother Miriam struggles to preserve Agnes's innocence, even if doing so means leaving intact her mental disorder, Dr. Livingstone, though, fights for the young nun's mind at the expense of her innocence. Dramatically, the conflict between faith and reason is perfectly convincing, largely because Plummer performs the role of Agnes with quivering openness and absolute accuracy. Something in her gentle, husky voice--genetic courtesy of Mother Tammy Grimes--seems tailor made for the part of the young nun. In her hands Agnes degenerates into neither a foolish, mooning saint nor an unbelievable madwoman.

Lee Remick, despite her experience and usual excellence, represents the weak link in the cast. Perhaps the role of Dr. Livingstone provides insufficient depth and variation for a good characterization. In any case, Remick's "scientific" composure seems more a lack of acting than a lack of warmth. Remick's performance improves as Dr. Livingstone's personal involvement with the case increases, but her mannerisms remain stylized, false and some how jarringly over-bright.

In terms of its ability to involve and to entertain Agnes of God deserves unstinting praise. The production is clean, neat and generally falls disappointingly short of its mark. Its full throated advocacy of faith and its lack of respect for ambiguity doom it to failure as a drama of ideas. The verdict of the fashionmonger on Agnes of God as a potential trend-setter should be negative. There may yet be life in the psycho-drama form, but it wants a Prince Charming, not this unevenly constructed work, to make it awaken.