Bach festivals are spreading: Bethlehem, Pa., is the godfather of them all in this country, but Berea, Ohio, will be 50 in two years, and Eugene, Ore., home of the University of Oregon, has been doing handsomely by Bach for a decade.

Lovers of Bach organ music in the Washington area have, for the past two years, seen signs in the Chevy Chase Presbyterian Church on the Sundays when the Bach Marathon was in progress: "Bach's Lunches." In Eugene they also have "Bach Suppers," and my favorite, hanging over the appropriate window: "Bach's Office!"

Eugene also has several other unusual and high-caliber attractions. First of all, they have had as music director for 10 years Helmuth Rilling of Stuttgart, in the Federal Republic of Germany. That latter designation is of particular importance these days. Cultural cooperation between the West and East sides of Germany is permitted today in only three or four areas. One of these is the Goethe Ausgabe, or complete edition; another is in the Neue Bach Ausgabe, the new edition of Bach's works. Rilling is one of four musicians on the committee for the Bach edition, two from each side of the line dividing West Germany from East Germay.

Some Washington music lovers, though not very many, will remember a wonderful Bach concert Rilling conducted in Constitution Hall in 1968. There were not many there to hear his Gaechinger Kantorei and Orchestra from Stuttgart, but those who did go heard Bach of the highest order. They will return in the season of 1980-81, undoubtedly to the Kennedy Center where they should not be missed.

What Rilling does in the Oregon festival is astonishing, and it has a fascinating historical precedent. His work there centers around the cantatas of Bach, all of which he is currently in the process of recording before the 300th anniversary year of 1985.

On eight of the festival's 13 days, Rilling presides over rehearsals and performances of eight different cantatas! He does this at the same time that he is preparing and presenting such other major works as the Bach "Magnificat," Mozart's C Minor Mass, and "The Seasons" by Haydn. His singers, about 50 of them, come from all over the country, and are skilled, experienced organists, choir directors, singers and conductors. It is interesting to remember how often Bach himself, in his great Leipzig years, wrote, rehearsed and presented theses cantatas, often at the rate of a new one every week for several years.

From among the conductors present, Rilling selects various ones to prepare and conduct specific parts of each cantata, with outstanding soloists and an excellent orchestra at hand to take care of every requisite. At the end of the afternoon rehearsal, which is usually preceded by a closed session in the mornings, those conductors who have best handled their assignments take their place on the podium during the public performances that end each afternoon's work.

This system provides unique opportunities for conductors to learn the subtleties of accompanying various kinds of recitatives, of balancing choral passages with orchestra, and of infusing the whole with the elusive but requisite element of style, a thing much argued these days.

For Rilling, as for any distinguished Bach student, the key to the problems of Bach's vocal works lies in the text. Most of the time when Rilling stopped a conductor it was to ask, "What do the words mean here?" It also happened, from time to time, when a less experienced conductor would stop the orchestra and say, "Take it again, please." Rilling would ask, "What do you want them to do differently?" For one of the rules of conducting is that you do not simply say "again" without telling the players what it is that needs changing.

In the years that the University of Oregon has presented these special summer seminars, Eugene's audiences for the cantatas of Bach have grown not only in size but in their response to, and love of these works.

Among this summer's American soloists in Eugene were soprano Arleen Auger, tenor Jon Humphrey and baritone Douglas Lawrence. Auger is not merely perfect in vocal style in Bach and Mozart, as well as in Handel, which she sang at the Kennedy Center serveral seasons ago, but is one of the finest singers to be heard anywhere in the world today. That she rarely is heard in this country is a strong indictment of the musical systems that operate to keep American Singers working in Europe. With singers like these and an expert chorus and good orchestra, Rilling is able to present excellent Bach (and Mozart and Haydn) this summer.

With his own background as organist and a conductor with training both in Germany and Italy, and experience in conducting all the large choral-orchestral works, he makes Bach a very vital, living matter. While his style and musicianship are impeccable, there is a warmth and personal feeling in his Bach that goes far beyond some of the cooler, more restrained approaches that are today enjoying a certain vogue. While it will be good to see and hear Rilling making music in Washington again before long, it would also be good to think that he might return in the anniversary year with the forces he commands at the festival. CAPTION: Picture, For 10 years, Helmuth Rilling has been music director of the University of Oregon's summer festival of Bach music.