ON SEPT. 8, 1971, Leonard Bernstein's "Mass," termed by its creator "a reaffirmation of faith" in a world of violence, opened at the Kennedy Center for the Performing Arts. This week it begins a two-week run to mark the Center's 10th anniversary. Bernstein, concerned about how "Mass" would sound today, finds it "as topical as ever."

Ten years ago, the concept was startling to some people: "MASS: A Theatre Piece for Singers, Player and Dancers. Text from the Liturgy of the Roman Mass/Additional Texts by Stephen Schwartz and Leonard Bernstein." The upheavals of the Vietnam war, the search for peace and the assassination of the president were implicit in its themes. Bernstein, who does not accept commissions for compositions, wrote it at the request of Mrs. John F. Kennedy.

Coming back to it a decade later, Bernstein said recently from his summer home in Fairfield, Conn., "I was expecting to have some disappointment, a feeling that it was dated." In fact, he found its relevance increased.

"Oh God, yes. Today the problems are cosmic. There are the neutron bombs and the MX missiles, the ethnic problems and the generation problem. And peace? In 'Mass' the 'Dona nobis pacem' is more urgent because everything is 10 times more parlous than before. So today we just have to do 'Mass' and do it better."

In its first decade, "Mass" has been performed in Austria, Sweden and England, Hawaii, the Philippines and Japan. It has been seen on television and has been presented in the regular repertoire of the Vienna State Opera. In this country it has been seen and heard from Connecticut to California and from Texas to the stage of the Metropolitan Opera.

"Mass" includes the entire text of a mass as it is performed every day in Catholic churches around the world. The additional texts are contemporary comments deriving from the traditional lines. As the Celebrant, the central figure of the "Mass," moves into the heart of the liturgy, his words and the choir's singing elicit differing reactions of doubt and disbelief from the street people who are watching or joining with him. Opposing the Celebrant's affirmations of faith is a kind of Nemesis-figure who sings "Non credo" -- "I do not believe."

Eventually, as the Celebrant seems to become increasingly remote from his people, donning the ceremonial garments for the celebration of "Mass," the people -- even his faithful choir and choir boys -- turn against him until he, in a violent outburst in a mad scene that runs nearly 15 minutes -- throws down the sacred elements, wrecks the altar and finally disappears from sight. Only after a long silence do voices of reconciliation begin to sound again, and "Mass" ends on a note of hope, with the traditional kiss of peace.

Is the Celebrant's role more difficult today? Is it harder for him to have faith than it was 10 years ago?

"No -- it may actually be easier today," Bernstein said. "Because the problems are greater, he may feel an even greater sense of urgency than he did 10 years ago."

Are there changes? "Yes -- in the text. None in the music, except for one cut, which has always been indicated as optional in 'The Word of The Lord.' I have decided to make it. Stephen Schwartz, sitting right here by this swimming pool last summer . . . said he had always hated some of the poetry, that he felt as if he had been showing off. Or that there were lines that were not clear, like 'Now is the age of gold.' What does that mean? . . . We made a long list of what we wanted changed."

Due to postponements and Schwartz' work in Hollywood, not all the changes were made. "So there will be this new production with some wrong lines," said Bernstein. "But he did manage to send a whole new version of the 'Confiteor.' And I made some changes. Things like 'There's so much I could show' in place of 'There are scars I could show.' I never liked 'scars.' "

"Mass" will not be Bernstein's only contribution to the Kennedy Center's 10th birthday party. Following the official celebration Saturday night, he will appear Sunday as piano soloist with the National Symphony under Mstislav Rostropovich to play the Mozart Concerto in G, K. 453, in the NSO's salute to the Center's 10th anniversary. "I have so much practicing to do!" he lamented.

Last March, when he made an audio-visual tape for the Kennedy Center's archives, as a recipient of the Kennedy Center honors, Bernstein said, "I love being in Washington and every time I come here it's like a holiday. It's not just a trip or a job conducting. There's something about Washington that turns me on. It always has . . . I feel a very deep connection with the city because I think I hammered the last nail into the Kennedy Center 10 years ago when it was being constructed." Few have contributed more of creative beauty to the Kennedy Center's first decade than the composer of the first music ever heard within its walls.