Benjamin Britten was what the English used to refer to as a “conshee” — a conscientious objector. He was safely ensconced here in America in the early days of World War II, but courageously decided in 1942 to return and face the worst of it with his countrymen. On Wednesday evening, the boy’s choir of St. Thomas Church presented the masterful piece he wrote aboard ship on that fateful journey home.

Biographer Humphrey Carpenter points out that “A Ceremony of Carols” is neither ceremonious nor, strictly speaking, does it contain any carols. The text is not about the Christ child per se, but rather an exploration of two types of pure innocence, before and after the fall. The story goes that Britten found a volume of medieval poetry in Halifax at the beginning of his voyage and set some of its verses to pass the time. He was also then working on two commissions, one from Benny Goodman and one from a harpist, and so had two harp manuals packed in his luggage.

There are really two distinct parts to the work. The processional and recessional are religious in nature, antiphons at the Magnificat of Second Vespers of the Nativity. Everything else is secular in nature and is itself bisected between the light and the dark. A brilliant harp interlude, one of Britten’s most gloriously modern pieces, is the line of demarcation.

The boys of St. Thomas — one of only four full time choir schools left in the world, with boys ranging between the third and sixth grades — created a stunning acoustical effect right from the start when they began singing in the wings. The Hodie Christus natus est acts loosely like an introductory theme that will be varied in the body of the work. The sound of the 21-voiced chorus led by John Scott, formerly the music director at St. Paul’s in London, was richly resonant and just a little less polished than an adult choir might be at its zenith. This very roughness added to the sense of innocence of the piece and played warmly against the campanilian but somewhat chilly timbre of the harp.

Britten writes primarily for the full ensemble, but there are the occasional solos, and one very clever duet that continually suggests that it is about to break out into a full-fledged round but never actually does. In what I suppose was an egalitarian gesture, the program did not list the individual soloists, only all of the names of the boys. Some sections worked better than others; among the best were the interplay of soloist and chorus in Balulalow and the boisterous spirit of This Little Babe. There is an unwritten rule in our profession never to criticize children, but I must admit to being surprised at how much waywardness of pitch was allowed by such a prestigious organization.

Harpist Anna Reinersman was fine throughout. She produced an interlude of pentatonic paradisiacal quietude that begged the listener’s rapt attention through sotto voce technique. Elsewhere she was always at the top of her game, providing rhythmic and harmonic grounding as necessary.

The most impressive of the carols is “In Freezing Winter Night,” a masterpiece of “it’s so old, it’s new” dissonance. Britten was especially adept at atmospheric music — think of the orchestral imitation of the cithara in The Rape of Lucretia — and his transitions in this chiaroscuro tour de force are astounding. On the words “The Prince himself,” he creates a chord as exquisitely shocking as anything ever conceived by Debussy, and the boys handled it with great aplomb.

Britten was a Mozartian type of prodigy, passing the exams of the Royal College of Music with flying colors at the age of 10. As a lad he wrote over 100 works, neatly cataloged with opus numbers. He had a lifelong affinity for the boy’s life and wrote much of its repertoire’s most inventive music, such as the children’s parts of the War Requiem. He even advanced the potential for the treble voice as a substitute for the female one and composed an all-male opera, “Billy Budd,” where boys take all of the top parts. In such child-directed works as “Let’s Make an Opera!” and “The Young Person’s Guide to the Orchestra,” he plied his nostalgic trade without ever a hint of condescension. He was in a very real sense a boy who never grew up. And thank God for that.