As millions of YouTube viewers know, the "Hallelujah Chorus" is even hotter than usual this year. The wave started with a flash-mob performance by the Opera Company of Philadelphia and hundreds of local choristers. Dressed as shoppers, they sang the best-known anthem from George F. Handel's "Messiah" oratorio at noon in the downtown Philadelphia Macy's, which was already decked out for the holidays on Oct. 30th.

Then came the Nov. 13th performance that sent this viral-video trend into overdrive, when 100 vocalists -- led by a young woman singing the opening hallelujahs into her cellphone -- shocked a food-court crowd in a Welland, Ontario, shopping mall.

There are online reports and rumors about similar "Hallelujah Chorus" sneak attacks in the marketplace. The key is that many onlookers know this classic by heart and can sing along without missing many beats.

These are strange scenes, but they would not surprise anyone who has studied the history of Handel's masterwork and its stunning popularity, especially among American believers, said Calvin R. Stapert, a retired music professor at Calvin College in Grand Rapids, Mich. He is the author of the new book, "Handel's Messiah: Comfort for God's People."

The Macy's performance was spectacular and the food-court performance was just as fascinating in its own way, he said.

"One part of me says, 'Wonderful!' It's thrilling. ... Then I look at the comments that people keep writing" at YouTube.com as they respond to the videos, said Stapert. "Some of them are so deeply moved that this anthem to their Savior is being sung in such a secular environment. Then there are others who make it clear that, for them, this is nothing more than ... a novel way of saluting a cornerstone of Western musical culture."

No one knows why "Messiah" has become so popular, noted Stapert, in his book. The work's omnipresence -- with performances in churches, civic centers and elite concert halls -- is probably the result of "musical, textual, social, religious and psychological factors that will never be completely unraveled." There is no precedent in music history for this phenomenon.

For starters, Handel is an unlikely hero for today's musical masses. He was a "reluctant eighteenth-century German Lutheran composer who would have preferred to continue writing Italian operas in Protestant England, a country that had no oratorio tradition until he 'invented' it. The rest, as they say, is history," wrote Stapert.

This musical form -- the oratorio -- was also a unique and at times controversial kind of art. Handel composed "Messiah" and many of his greatest works in a cultural no man's land between the music common in sacred sanctuaries and the lively, entertaining, operatic works that were popular in theaters and concert halls. Nevertheless, most oratorios were based on the lives of biblical heroes and early Christian saints.

Then there was "Messiah: A Sacred Oratorio," which was composed in 24 days and performed for the first time in Dublin in 1742 and a year later in London. The libretto covered the drama of the full Christian liturgical year, yet the work was never intended for church performances. Handel originally composed the work for approximately 24 skilled singers and 24 instrumentalists.

Today, "Messiah" is often performed with choruses consisting of 100 singers or more and orchestras of every imaginable size and composition. In many performances, amateur performers are forced to cut the tempos of Handel's mercurial, dancing choruses until they resemble lumbering musical stampedes.

To state the matter bluntly, noted Stapert, no complex work of classical music "has survived, let alone thrived, on so many performances, good, bad, and indifferent, by and for so many people year after year for such a long time."

Now, the most famous anthem from this Christian masterpiece has reached the true public square of our age, in the same mix as "Jingle Bells" and "Rudolph the Red-Nosed Reindeer."

"You have to ask," noted Stapert, "if many people are really listening to the words. After all, who is this 'King of Kinds and Lord of Lords'? ... You have to think that the cultural police would be out in a matter of minutes to shut this down if people were paying attention to this profoundly Christian work that is being sung right out in the open, in a mall. Has the 'Hallelujah Chorus' become so familiar that people cannot hear what it's saying?"