Washington has, in recent years, evolved into a major center for the performance of a wide variety of the works of Bach. With its notable array of organist-choirmasters, fine pipe organs and top choral groups, and the experts in the performing arts division of the Smithsonian, there is no music of Bach that cannot be well managed by musicians in this area.

In addition, several annual events have become established that adds special interest for Bach-lovers here. The Bach Organ Marathon at Chevy Chase Presbyterian Church has become a success in two years; the Washington Bach Consort regularly presents programs on which the entire catalogue of about 215 Bach cantatas is gradually being offered. And the annual Bach Competition for Pianists is coming June 6-8.

Tonight at 8 p.m., the Church of the Ascension and St. Agnes at Massachusetts and 12th streets NW will begin its 15th annual festival of music by Bach. Each program, nightly through next Sunday, is open to the public without charge. This year's festival offers a kind of overview of Bach as the world approaches the 300th birthday of the Leipzig cantor, a major milestone to be celebrated five years from now. The moderate-sized church is one of the acoustical glories of this city, with an intimate warmth that has made it, from time to time, a favorite place for young seminarians to try out their preaching voices. (The only problem is that the aspiring preachers will be lucky to find as receptive a place for their sermons later in life.)

Drawing on the musical resources of the entire Washington area, the festival opens with the Alexandria Choral Society under the direction of Douglas Major. They will sing Cantata No. 37, "Wir da glaubet," and the Missa Brevis in G Major. One of four short settings of the Mass in the Lutheran tradition, the G Major Missa is, like its fellows, drawn almost entirely from music written previously for various cantatas. Bach, like his illustrious contemporary Handel, knew when, where and how to borrow. On Tuesday evening the Ad Hoc Singers, directed by Louise Lee, with Michael Donaldson and David Richie as Soloists, will present one of the solo cantatas for bass, No. 56, "Ich will den Kreuzstab gerne tragen," and the famous one-movement cantata No. 131, "Aus der Tiefe."

The Washington Consort, with its conductor-harpsichordist Reilly Lewis, will perform the concerts on Tuesday and next Sunday nights. On Tuesday there will be sonatas and the Third Orchestral Suite, plus the Italian Concerto and the Italian cantata, "No sa che sia dolore" sung by Doris McLaughlin. On Sunday night Lewis is attempting a daring feat: the perfor- [TEXT OMITTED FROM SOURCE]

When Mozart heard the motet, "Singet dem Herrn," in Leipzig in 1789, he wrote, "My whole soul seemed to be in my ears." Nowhere in all of Bach is his feeling for architectural balance more brilliantly illustrated than in these extraordinary compositions. It is fortunate that Lewis has both singers and instrumentalists in his Bach Consort for this program, because both are required for their proper performance. It is a strange fact that until fairly recent years, it was an established custom with collegiate and church choral groups that often took up one or another of the motets, invariably to sing them completely unaccompanied.

There is irrefutable historical evidence that this kind of practice was never in Bach's mind. In the first place, a cappella singing was very rarely heard in Germany in Bach's day. Furthermore, there exists the original set of both string and wind instrumental parts for the motet, "Der Geist hilft unsrer Schwachheit auf," in which Bach consistently doubles the voices. From these two facts alone, it is obvious that the motets were never meant to sound forth in naked, unadorned choral splendor. There is also the weight of the entire performance rationale underlying all baroque music: that is, the use of either organ or harpsichord plus a string bass undergirding all instrumental and vocal music of that period. It is only reasonable to assume that what Bach specifically provided in one case, he anticipated would be repeated, at least to some considerable degree, in the others.

A study of the music of the widely differing motets makes these conclusions inarguable. The mighty fugue that forms the central stone of the great arch of "Jesu, meine freude," and that overpowering four-part fugue that closes the eight-part motet, "singet dem Herrn," are choral structures that demand iinstrumental support. Anything less is a mimicry of Bach.

On Wednesday night an instrumental ensemble will play the Trio Sonata from the Musical Offering, to be followed by organist William Crane in the six Schuebler Chorales. Two duplications will occur in the festival: The Glee Clubs of Saint Albans and National Cathedral Schools, directed by Richard Roeckelein, will close the Wednesday evening concert with the sixth of the motets, "Lobet den Herrn, alle Heiden," and Cantata No. 150, "Nach Dir Herr, Verlanget mich." The same cantata will be heard again on Friday night when Kerry Krebill conducts the Musikanten is a program that will include organist Robert Grogan playing the Toccata, Adagio and Fugue, and Jody Gatwood in one of the solo partitas for violin.

Keeping to its annual custom, the festival will mark Ascension Day with a special Evensong and Benediction on Thursday, sung by the choir of men and boys from Washington Cathedral under the direction of Richard Dirksen. The Saturday night program will bring the Cecilian Consort with James Scribner as conductor, flutist Judy Moore playing an unaccompanied partita, followed by Cantata No. 4, one of the best-known, "Christ lag in Todesbanden." And on Sunday night, the six motets.

At various times and in different places and ways, Washington is gearing up for that great Bach Festival to come in 1985. This week in the church of the Ascension and St. Agnes you can get a glimpse of glory to come.