In the 17 years since the Imperials began performing in small southern churches, gospel music has gone through a tremendous upheaval. Many people compare its phenomenal growth to that of country music's over the last 10 years. The Imperials used to perform for after-service suppers; now they're in the midst of a multicity tour, playing large arenas (Constitution Hall tonight, for instance), complete with a five-piece band and elaborate sound and light systems; they're the first Christian group to approach a tour as a secular venture, but "our's is mainly a Top-40 sound . . . played in an acceptable fashion," explains Armand Morales, founding member and leader of the four-man group. "People do have a fixed image in their mind of what gospel music is." Smartly dressed, with gold chains around his neck and wrists, Morales resembles an older, softer Robert Blake, with the cool, good looks of a pop star.

Tenor Dave Will is an ordained minister who still gives a prayer and a call for commitment mid-concert, but the Imperials' new lead singer, 28-year-old Paul Smith, is considered something of a matinee idol in the once-staid and ultra-conservative Christian music industry that is beginning to look more and more like its secular counterpart. The old message is being delivered in slick, new packaging.

Recently, gospel music sales surpassed jazz and classical music sales, and much of that growth is due to the younger, less conservative and less doctrinaire listeners to whom the the Imperials reached out long ago. Their base is more middle-of-the-road now, but that initial outreach was difficult for many to accept. As Morales says, some people "had problems dealing with that kind of music rock, pop in their personal lives and now they were facing it in our music."

The Imperials' latest record, "Priority," which has sold 450,000 copies, is full of cascading brass lines, an incessant backbeat, rock guitar and harmonies reminiscent of Earth, Wind & Fire and the Doobie Brothers. The group, winner of three Grammys and a dozen Dove awards, has even been called the Bee Gees of Christian music. "Priority" was produced by Michael Omartian, a Grammy winner more than once for his work with Christopher Cross. The group's latest producer just finished an album with The Jacksons. The Imperials do 150 major concerts a year and appear on network television (the group says Barbara Mandrell wants them for her show); and they're even planning a cruise in January that will be filmed and syndicated for television. The power of positive pop certainly seems to be with the Imperials.

It wasn't always that way. As gospel music started to break down its fixed image, the Imperials were there. The original members started out as traditional southern gospel quartet, but having grown up exposed to rock and pop influences, Morales says, they "moved into a contemporary style. The band was brought up a little heavier, the high tenor and low bass weren't featured so much. It was more like a Top-40 sound." It was also a radical idea at the time, but, says Morales, "We had a conviction to reach the youth market, which we felt our kind of music wasn't doing at that point. You're dealing with how people think about God. For young people brought up in the Christian community, it's a positive thing for them to have music they can identify with."

The battle between secular and sacred rages on today, fueled by a growing interest in religion, marketing techniques applied to the gospel music field and, most of all, a tremendous surge in the number of young performers working in the once-moribund field. Things are still broken down along racial lines: The black side is called gospel, the white side either spiritual, inspirational or contemporary Christian. Though there's very little crossover, the Imperials were the first white southern gospel group to hire a black singer, Sherman Andrus.

As time went on, the group did grow in popularity, no more than in 1966, when they recorded with Elvis Presley on his "How Great Thou Art" album. For several years after that, they worked with Presley in Las Vegas or on his concert tours, and were "always introduced and billed as a gospel group" (though they did occasionally back up other Presley music). Morales is troubled by the revisionist portrait of Presley now in vogue.

"He was always very good to me and the Imperials. He treated everybody with a lot of fairness, let everybody live the way they wanted to without pointing a finger. He wanted every man to be his own man." Morales particularly resents the image of a profane Presley, insisting Elvis always had a "twofold enjoyment area. After many concerts we would sit up two or three hous singing gospel while he wound down. He had a deep love for that. On the other hand, he loved the other music very much. You can like both, there's nothing wrong with that."

Exposure with Presley (and for six years with Jimmy Dean) brought the Imperials offers to work the big hotels and concerts. The group has never toned down the gospel message in favor of a half-and-half pop mix -- despite the tremendous success of the Oak Ridge Boys, for years the Imperials' only serious competition. "We never considered it," Morales insists. "We're ministers who sing gospel music and we've always kept that conviction in our heart. It's never even been a temptation because our roots are so strong and we feel good about the choices we've made."

Country music grew partially because it appealed to adults in a way rock generally doesn't; the same thing could happen with contemporary Christian music as it reaches out to people through religion. "We preach a basic, fundamental gospel which every Christian can go along with," Morales says. "We have a strong belief that Christianity is not a downer at all, that it's a good life, one that has a lot of joy in it." In the born-again '80s that belief is increasingly visible as more country-and-soul artists, who frequently come out of the gospel tradition, share their faith. Born-again artists like Bob Dylan and Donna Summer may get the press, but it is leaders like the Imperials who opened the field and still dominate its sales and charts. "We are here first and foremost to serve God," Morales says.