Sometimes you just don't see a significant piece coming.

The composer was relatively untried, worked in an oddball genre, and explored an unexpected subject - a German pastor and theologian who battled Nazism and died for it. Nonetheless, Thomas Lloyd's choral theater piece Bonhoeffer, premiered by The Crossing choir on Sunday at the Philadelphia Episcopal Cathedral, was a fully realized 70-minute work and a breakthrough for all concerned.

The piece's artistic significance springs from (but doesn't trade on) its subject, Dietrich Bonhoeffer, the influential Lutheran theologian who was part of the "Stauffenberg Plot" to assassinate Adolf Hitler . Texts were drawn not just from Bonhoeffer's protests against religious institutions capitulating to the Nazis but letters to his ambivalent fiancée, Maria von Wedemeyer, who after Bonhoeffer's 1944 execution lived in Bryn Mawr.

In his program notes, Lloyd, music director of the cathedral, envisioned "choral theater," in which male singers in street clothes melted into the audience at times and made dramatic entrances at others. The musical envelope was filled out by a small chamber ensemble and three female soloists. With a loosely traced scenario conveying Bonhoeffer's inner journey, no one voice was assigned to a particular character.

The main priority was effective articulation of the words, whether spoken (Nazi dictums that don't deserve music) or choral recitatives. More formal sections had such a strong rhetorical element that they sounded half-chanted.

Harmonically, the piece didn't play by the usual major key/minor key rules but explored a more emotionally neutral world (like Machaut's 14th century Mass of Notre Dame) that kept the dramatic content safe from cinematic sentimentality. Yet even when Bonhoeffer was chatty and cerebral, Lloyd pumped up his words with vigor and exterior conviction.

Quotations of pre-existing music played a major role here. One of the strongest set pieces, "Blessed are the poor in spirit" has an effective collage moment with Paul Robeson's recording of "Sometimes I Feel Like a Motherless Child" (Bonhoeffer had studied in New York, taught Sunday school in Harlem, and loved spirituals). Other quotes (Schubert songs, Schütz religious works) were embedded in the textures, often with an emblematic commentary, and always working wonders as purely musical counterpoint.

Herein lies the piece's hallmark: While it effectively airs many philosophical questions that keep your mind busy long after the performance, it is never weighed down by them, and is rich in musical substance. This is made apparent by the typically rock-solid singing of The Crossing - even when singers and director Donald Nally were physically mobile during the performance. Tim Early's choreography was lost to me due to bad sight lines. Did the music really need it?

More palpable were the instrumental ensemble's elegance and passion - as one might expect, given that plenty of solo moments were taken by cellist Ulrich Boeckheler, whose father had been a Bonhoeffer classmate.