For believers, the miracle of Jesus may have been the overt message of Anonymous 4 Friday night in its annual Christmas pilgrimage to St. Mark's Church on Locust Street. But for those of us for whom music is the religion, the bigger miracle is the group itself, which practices a kind of aesthetic bliss and technical perfection all the more dear in a deeply flawed world.

Trafficking mostly in music derived from ancient sources, the a cappella quartet has sold more than 1.5 million albums in its two-plus decades. If that seems unlikely, put it down to the group's connection with a number of overlapping constituencies - not just the musical and religious, but perhaps also a large group of waking aesthetes for whom meditative, soaring vocal sonorities are a great comfort. Either that, or the number of music historians specializing in medieval manuscripts has been vastly undercounted.

Anonymous 4's gifts are even more starkly apparent in concert. Hosted by the Philadelphia Chamber Music Society, Marsha Genensky, Jacqueline Horner, Susan Hellauer and Ruth Cunningham took turns on the high part or in solos. Repertoire for this program, called "The Cherry Tree," named for the miracle ballad of Joseph and Mary, is headed for a CD to be released in 2009. In works from the 15th century to traditional British-American tunes, the solo work underlined the best of all possible reasons for the group's homogeneity. Turns out they're not naturally like-voiced at all. Each has a distinct tone and style in solo work, so the fact that they sound so much like one another in ensemble is the conscious result of skill and effort work.

In "Lullay my child: This enders nithgt," Cunningham's sound was plain, even girlish. Horner is the home of purity in the high register, utterly vibrato-free yet unfailingly dead-on in tune. Hellauer often takes the low notes, girding the group with her rich sound. And in one carol, Genensky ventured into folk-tune inflections distinct from her ensemble sound.

As impressive as their polyphonic work is, it's those pieces and passages in which all sing the same notes that nestle in the consciousness most durably. Recently, a certain hypnotic melody surfaced in my head and hasn't gone away since. It's the "Hymn: Intonent hodie" from

Legends of St. Nicholas

, the group's release of a decade ago. After Friday, I'm looking forward to what melody may spring next, a volunteer souvenir from a concert heard long ago.