AS the church once owned the sacraments, drama once belonged to the theater. Now that the movies and television (even radio) have exploded that ancient monopoly, is the theater the anachronism some think it is? Economically fragile and technologically backward, it hangs on, but how? Does anything but nostalgia and entrenched habit sustain its feeble pulse? Or does it, on the contrary, still serve some unique and vital purpose?
''Metamorphoses,'' an Off Broadway play written and directed by Mary Zimmerman, might change the mind of anyone who supposes that it does not. An anthology of myths of transformation, this play of miracles derives chiefly from Ovid's endless, ragbag poem of that name, dating to the reign of the Roman emperor Augustus. The poet's ambition, as stated in the opening invocation, is to catalog transformations on every scale, from the microscopic to the cosmic, from the creation of the world up to the moment that the stories are told. Happily, Ms. Zimmerman takes the program no more literally than Ovid did.
''Metamorphoses'' is theater through and through. Like a lepidopterist with a pin, a filmmaker memorializing the production might preserve certain colors and contours of the specimen. But its life is on the stage. (Currently at Second Stage until Dec. 30, the production is scheduled to have a Broadway run when it moves to Circle in the Square, where previews begin in February.) From the title through to its candle-lit coda, ''Metamorphoses'' depends entirely on the interchange between living players and a living audience. Transformation here is not merely a theme, but the mystery at the core of existence, as it is at the core of theater. Props change, costumes change, sets change. Most significantly, we change.
For good reason, Ms. Zimmerman sets the action around and within a pool of water. ''Being named different things, the water becomes different things, besides being a symbol of transformation transculturally, as an element that both purifies and corrupts,'' she said recently by telephone from her office at Northwestern University in Chicago, where she teaches performance studies. ''Everyone changes. Change is so necessary, yet so painful. It's the condition of human life. There will be transformation, and you will grow old.''
A mournful message? Not invariably. Ms. Zimmerman's shape-shifting cast of 10 brings before us tragic kings and comic commoners, reprobates and heroes. Behold King Midas with his fatal golden touch, chimes sounding at his footfall. Orpheus, losing Eurydice over and over with his backward glance. The godless Erysichthon, his hunger so insatiable that in the end he has at his own foot with fork and knife. The coy Myrrha, aflame with sudden lust for the father who begot her. In the richest sequence of all, Poseidon in his wrath rains down torrents from a bucket, capsizing a toy ship, while elsewhere in the pool, a strapping sea spirit drowns Ceyx, the life-size captain, in the breakers. The transformations that ensue -- into trees, birds, streams or statues -- are by turns curses, blessings and sometimes, most magically, the outward sign of mercy, of a judgment suspended.
Anyone who has been to the movies in the last few years can easily imagine such sequences as rendered by the seamless enchantments of animatronics. Ms. Zimmerman achieves them by the most transparent of means. Words tell the story, strategic bits of mime add their thunderbolt or murmur of suggestion, and the spectator's wish to believe does the rest. We see what is and we see what is not. As for the meanings, they do not concern the eye alone.
For a searching meditation on such matters, we may turn to Peter Brook, that most restless of gurus, who has taken up the subject often, notably in his book ''The Empty Space.'' Emptiness is, of course, a relative term rather than the absolute Mr. Brook pretends. (We need air. We need light. And what space is ''virgin'' -- free of association?) Still, any child will grasp instinctively the principles that Mr. Brook belabors. In the Young Vic Theater Company's ''Grimm Tales'' at the New Victory Theater three seasons ago, not one little voice piped up to ask what was going on when the actors spread a sheet over the body of Cinderella's mother. In the twinkling of an eye, the dead woman became the statue on her monument, to the hush of total, untutored comprehension.
But if the transformation of persons is what interests you, how can the stage compete with the movies? Consider Alec Guinness, skipping from role to role in ''Kind Hearts and Coronets,'' now a twerp, now a dowager, now a mumbling cleric, now a bully among the landed gentry, each one realized to perfection in every detail of dress and makeup.
Actually, that simulacrum of reality comes at a price. Choose as we may to ignore the fact, presence on the screen is by definition an absence. The actor is never there. All is deception. Thus the cinematic illusion depends absolutely and unconditionally on the unwavering suspension of disbelief. Whereas the theatrical illusion, sketchy and fragile as it seems, presupposes a presence. The actor is there, capable of taking us anywhere, of being anyone. As transubstantiation requires not just an act of faith but also real bread and wine, theater requires the living presence of the actor.
How infinitely various live actors are: in ''A Tuna Christmas,'' a Broadway hit in the mid-90's, Joe Sears and Jaston Williams morphed into 22 citizens of Tuna, Tex. (young and old, male and female). Last season, Sean Campion and Conleth Hill opened on the same block, pulling off a similar act in ''Stones in His Pockets.'' In ''A Christmas Carol,'' Patrick Stewart, working solo, conjures up a Dickensian gallery numbering three-dozen-plus (this well-traveled tour de force will return to Broadway for eight benefit performances at the Marquis Theater, Dec. 24 to 30). In a Molière double bill by the Roundabout Theater Company in 1995, without having disguised their faces, Brian Bedford and the entire cast came back from intermission so altered in bearing, dress and voice that they seemed actually to be different actors. Reversing the arrow that same year, Brian Murray, Jim Dale, Martin Rayner and Tom Beckett parceled out the role of Henry Pulling among them (and divvied up all the other parts) in Giles Havergal's ingenious adaptation of Graham Greene's ''Travels With My Aunt.''
Ian McKellen, currently starring on Broadway in Strindberg's ''Dance of Death,'' is another wizard of the quick change. His one-man show ''Acting Shakespeare,'' performed without props or makeup, featured 20 characters, among them both Romeo and Juliet in a single scene. Equally memorably, his Salieri, in Peter Shaffer's ''Amadeus,'' transformed himself in the twinkling of an eye from a doddering invalid to a dashing courtier in Vienna a half century before. The play was Mr. McKellen's first Broadway triumph, and people who saw it talk of it still.
''That took various bits of technique and trickery and sleight of hand and deceiving of the eye,'' Mr. McKellen said recently, reconstructing from memory the entrances of supporting players, light cues and costume details that supported his performance. ''But for it to be convincing, I had to feel myself to be old, just as I then had to feel myself to be young and living in 18th-century Vienna. It's called acting.''
How difficult are such changes? ''Actors can turn it on, you know,'' Mr. McKellen replied. ''They're used to that. The director in the theater calls the start of a scene, and you're in character. On a film set, they call 'Action!' and suddenly you're a different person. The audience doesn't often see it happen. It's not often required. But if people wonder how it is achieved, they only need to think of what happens in real life when you have good news or bad news. The new impressions and feelings are instantaneous. You will look different, feel different and potentially speak differently than you did a second before. It's part of human experience.''
AS chance would have it, Mr. McKellen was on his way to a screening of ''The Lord of the Rings,'' in which he appears as the benevolent sorcerer Gandalf. Mostly, the actor's mind seemed to be on Gandalf's battle with the Balrog, a monster of fire. Mr. McKellen would be setting eyes on his adversary for the first time.
''Cinema magic is getting more and more commonplace as it gets more and more complicated,'' he reflected. ''To me, the monster was just a yellow tennis ball on top of a stand, to give me my eye line. There's hardly any magic left in the cinema, is there? The particularly thrilling thing in the theater is that the audience colludes. They allow magic to happen, know it can't have happened, and are full of wonder that it did happen. If they thought about it for a second, they'd know it never happened at all. That's the thrill, the impertinence of a transformation in the theater. But as far as storytelling goes, a transformation will only have huge impact if it's an essential part of the story.''
Not long ago, Mr. McKellen saw ''The Winter's Tale'' in London. It is a play he has acted in more than once. Yet he, too, was overwhelmed when the supposed statue of the dead Hermione stirred and came back to life.
''That's the most wonderful example,'' he said. ''Even for an audience that knows exactly what is going to happen, when the music starts playing -- oh, Shakespeare knew a thing or two about staging -- the effect is achieved. And the effect is to stop the heart before the tears start flowing.''
Back in Chicago, Ms. Zimmerman touched the same string. ''What attracts me to theater is the collusion between the audience and what is happening onstage, believing what they know not to be true. Shakespeare names it in the prologue of 'Henry V': 'Can this cockpit hold/ The vasty fields of France? Or may we cram/ Within this wooden O the very casques/ That did affright the air at Agincourt?' ''
''He's throwing down the gauntlet. 'We're going to be so good at what we do,' he says, 'we, the actors and the audience -- that collectively we're going to understand this language that is theater.' And it's not just verbal language. That's the pleasure of the theater: the slippage of difference between what you're seeing and what you know you're seeing. That square box can fill up with almost anything. In movies, it's a special effect. Here, it's achieved by an act of faith.''
Hearts stop, tears flow. From ''Metamorphoses,'' we know in our bones that some sorrows pass away (not all), some wounded hearts (not all) do mend. What we have witnessed in characters who have lived and breathed with us can happen for us, too. That is the theater's timeless promise. It is never out of date. But in chaotic times like these, we have a special need to hear it.