Sarah Caldwell's Opera House was only half-filled here for the American premiere of Peter Maxwell Davies' opera "Taverner" -- perhaps the year's most significant operatic event in America.

But if the conservatives in Boston chose to ignore this avant-garde event, those who attended were treated to a provocative evening and responded with long, loud applause.

Postponed twice because of Caldwell's chronic problems with money and health, "Taverner" proved to be well worth the long wait. Its impact in this production is stronger conceptually and visually than musically, but the opening night worked well in all departments. Musically "Taverner" may lack some of the eccentric vitality of Davies' shorter stage works -- for example, the "Eight Songs for a Mad King" -- that were performed in Washington last year. But it is highly efficient, functional theatrical music, used to energize point and accent the words, and occasionally rising to heights of great lyric intensity. The libretto (also written by Davies) was clearly the composer's prime preoccupation in this opera. It contains a lot of (usually grammatical) Latin as well as its basic English, and surtitles were used for both languages.

Theatrically, this well-staged production is a visual extravaganza, making effective use of props and costumes on what is essentially an empty stage without scenery. The colors of the staging are developed as carefully as the plot and music. The first act opened with a courtroom scene staged in sober black and white; by its fourth and final scene, it had risen to a climax of pure fantasy -- a riot of tortured, nightmare-like color highlighted by dozens of little red devils who dance across the stage while the title character sells his soul to the powers of darkness. The second act begins, once again, on a note of visual sobriety and reaches its climax and conclusion with a scene of memorable horror. As the stage fades to its final blackness, the last remaining light is the red glow of a pyre where a Catholic abbot -- Taverner's chosen enemy -- is being burned at the stake.

On opening night, Caldwell conducted a performance that was, musically, more careful than energetic, but the music was clearly in capable hands.The most impressive performance was that of Alan Oke, a light baritone who sings several roles -- most notably that of Death, a figure who dominates the opera's action through most of the evening. Tenor John Moulson was also impressive in the role of John Taverner, and there were noteworthy performances by Raimund Herincx, Andrew Galacher, RoseMarie Freni, Noel Velasco and Jeffrey Gall.

At the beginning, the chorus represented a group of medieval monks, but by the end it had become a group of spectators in modern dress commenting on the action. It sang and acted effectively throughout. The Opera House, formerly a movie theater in downtown Boston, has remarkably good acoustics. Nearly every word of the sung text was intelligible, even without looking at the surtitles. The orchestra, in a deeply sunken and partially covered pit, never once outbalanced a singing voice; if anything, the singers tended to dominate the orchestra.

"Taverner," like much of Davies' works, occupies a stylistic no man's land between realism and surrealism. Its cast of characters includes two historical figures: King Henry VIII of England and John Taverner, a composer of religious music during Henry's reign, whose surviving works are not numerous but of very high quality.

There are other real characters, but the cast also has two archangels, Michael and Gabriel, as well as God the father (a large puppet with a countertenor voice) and Jesus Christ, a baritone who also sings the more prominent role of Death. There is only one woman soloist, a mezzo-soprano, who sings the role of Taverner's mistress but also doubles as the Virgin Mary. The action shifts fitfully between realistic exposition and surreal fantasy, dreams and visions.

Whatever else it may be, "Taverner" is the most complex musical statement ever made on the Reformation -- specifically, the Reformation in England, which is the chief dynamic element in its plot.

The treatment is impassioned and, on the whole, impartial. The characters are not divided into good guys and bad guys; every character of any importance is a bad guy. This is particularly true of Henry VIII, but leaders of the old religion do not fare much better; they are represented as proud and closed-minded, hiding their greed under pretenses of piety. They are men who are willing to kill in the name of divine love, and as the opera opens, they are about to do so.

The first scene presents the trial of "John Taverner, musician, blasphemer, corrupter, heretic" by an ecclesiastical court, which condemns him to be burned at the stake. But he is rescued at the last minute and, in the new order established by King Henry, becomes a persecutor of those who persecuted him.

In the final scene, a grim, ironic echo of the first, he burns at the stake the abbot who had condemned him in the first scene. This time, there is no rescue. Taverner has destroyed his enemies but, in the process, he has also destroyed his own soul as embodied in his ability to compose music. This is made clear at the opera's emotional and psychological turning point, the final scene of Act I. In this surrealistic sequence, Taverner undergoes his dark night of the soul, gives his allegiance to Death and renounces everything he had been -- his faith, his love, his music -- to have his revenge.

If the opera has any moral, it may be that God is better worshiped by making music than by establishing institutions with creeds, rules and power structures.