NEW YORK — Carl Drews still remembers the first time he heard the “Hallelujah” chorus. His parents had taken him to see “The Greatest Story Ever Told,” the movie about the life of Jesus with a soundtrack including the famous chorus from Handel’s “Messiah.” It made the little boy feel that heaven was a place “where you sing the ‘Hallelujah’ chorus forever and ever.”

Years later, on a Habit for Humanity project in Nicaragua, the longtime choir singer sang the stirring words from memory under the newly built roof of a house, banana trees swaying in the breeze, with two other volunteers on his work crew.

This December, Drews, a 49-year-old software engineer, will participate in the 27th annual “Messiah” sing-along in Boulder, Colo., one of hundreds of such events across the country in which an unrehearsed audience performs as the chorus in George Frideric Handel’s baroque masterpiece.

“It’s just really fun to be with people singing their hearts out,” Drews said a few weeks after rehearsals began in Boulder for the core group of singers who support the audience. “And I am a Christian so I am singing what I believe.”

But you don’t have to be a Christian to love “Messiah.” Tens of thousands of Americans from all different social and religious backgrounds will gather in churches, concert halls and living rooms beginning in mid-December to sing all or parts of the 21⁄2 hour oratorio.

Many will have had some musical training, but others will barely be able to carry a tune.

In Chicago, some 7,000 people are expected at two performances of the Bank of America Do-It-Yourself Messiah, which will be performed at the Harris Theater for Music and Dance in Millennium Park. As many as 3,000 people will pile into New York’s Avery Fisher Hall when the National Chorale hosts its 42nd Messiah Sing-In. About 200 people will show up for each of three concerts in Boulder, and thousands more will participate in similar events at points in between.

The origin of the sing-along is a little murky. Graydon Beeks, a music history professor at Pomona College and president of The American Handel Society, says sing-along Messiahs seemed to catch on big here in the late 1960s and early ’70s. He remembers going to one such event with about 50 people when he was a college student in the late ’60s — but that was in the home of a family friend who was a professional pianist.

Martin Josman, the founding director of the National Chorale, claims to have started the “Messiah” sing-along in the United States when he trademarked the name “Sing-In” for his annual event at Lincoln Center. He says he came up with the idea to draw attention to choral singing, and chose the name “Sing-In” because the late ’60s was the era of “sit-ins” and “love-ins.”

“Messiah” holds an unusual place in music history because it has been performed continuously since Handel wrote it in 1741, according to Christopher H. Gibbs, a music professor at Bard College. It has remained famous since its debut in Dublin in 1742 and has been adapted in various ways — including by Mozart in 1789 for a bigger orchestra — to stay in the repertoire.

Part of the reason for its enduring popularity is that Handel was used to writing for the theater. “He knew how to make something moving, theatrical, exciting,” Gibbs said.

Perhaps none of the more than 50 movements is quite as thrilling as “Hallelujah,” when the audience rises and the chorus begins to sing, “Hallelujah! Hallelujah! For the Lord God Omnipotent reigneth.”