The congregation was assembled, the altar was prepared, the priests had donned their vestments, and last Sunday's evening mass was about to begin at St. Francis Episcopal Church in Potomac.

When the opening chord was struck, the sound was mixture of guitar, bass and piano, and the music was that of the Beatles.

The thumping beat of rock music filled the large sanctuary, "reminding us that Christ is here tonight," the Rev. William Tully said.

The 175 persons in the congregation did not clap with the beat of the music but, when the mass ended, the four persons playing the music assembled on the altar steps and received a standing ovation.

"Joy" is the main reason that Beatles music is appropriate for Christian worship. Tully said in an interview after the mass. He likened the British rock group and its search for truth and expressions of joy to St. Francis, for whom Tully's parish is named.

"St. Francis took profane songs and made church music out of them. There have been very few people who get close to the truth. St. Francis was onc. Probably, the Beatles are among them," Tully said.

"They (the Beatles) not only told the truth, they touched the truth that is in s us.

"John Lennon supposedly once said, 'The Beatles are more popular than Jesus Christ.' I say listen to what they did, not what they said. Love and joy are not easy to find, so they went off on their own," Tully said.

The congregation, half of which appeared to be under age 25, listened to the songs "Yesterday" and "Day Tripper" before the opening dialogue: to "Let It Be" during preparation for the eucharist; to "Hey Jude" while receiving Communion, and to "Hello, Goodbye" after the final blessing.

"Let It Be" was chosen for the eucharist, Tully said, in answer to the Christian controversies concerning what physcially happens to the host and wine as they are blessed."We wanted to let it (Communion) be whatever it is to each person," Tully said.

A rock mass featuring Beatles music was undertaken, Tully said, to provide another way to reach young people, many of whom Potomac churches can "have for th the asking," Tully said.

The music like the Communion bread, was made by students of Winston Churchill High School in Potomac. Many of those present were Churchhill students and members of other churches who responded to posters displayed at Churchill.

At a reception following the mass, 16-year-old Andrea Mion's enthusiastic reaction seemed to be representative: "I liked it. Church doesn't have to be the way you think of it. There can be different kinds of masses."

An evening rock mass is scheduled on the third Sunday of each month at St. Francis.