The National Symphony's four-week Bach Festival closed on Saturday night in the Kennedy Center with unsurpassable greatness as Karl Richter conducted the B Minor Mass.

The orchestra played very well, allowing for the problems that invariably plague high trumpets in this music. Richter found Robert Shafer's Oratorio Society much to his liking in their response to his very personal direction. Their tone was admirable, though here and there sounds of fatigue could be heard, and no wonder.

But the greatness of the music emerged throughout the evening in ways that constantly reminded the listener of the supreme genius it contains.

Richter was often surprising. Well chosen tempos were unexpectedly slowed down not only at the ends of episodes, but in their very midst. There were what today seem rather old-fashioned touches it ritards which were not dictated by the music or what was about to follow. But the total concept was that of a master with strong views, and Bach does not suffer seriously with this approach.

Maureen Forrester, busiest of the vocal soloists, was astonishing in the "Laudamus Te," in her technical control, and a marvel in sound in the duets and final "Agnus Dei." Benita Valente was her perfect partner in every aspect. David Kuebler sang the tenor lines easily in a way that enhanced their meaning. Paul Plishka's bass was a joy in both arias, though the height and agility of the second one drove him a bit.

Ransom Wilson's flute and Miran Kojian's violin joined the horn in enriching the instrumental solos.

The use of a tiny chamber organ in the presence of a chorus of over 110 voices was not only an unwelcome touch of preciosity but proved the sole disappointment of a performance.