Leipzig, East Germany — The 80 cherubs in the 7 1/2-century-old St. Thomas Boys' Choir breathe, eat, and live Bach. They sing Bach a cappella on alternating Sundays (in choirs of 40 ) in the church where the serene paterfamilias Johann Sebastian was cantor from 1723 to 1750. They weave his motets - sacred contrapuntal choral compositions - on Fridays and Saturdays in St. Thomas's as well. Then each year they perform the Christmas Oratorio and one, or sometimes even both, of the Passions (St. Matthew and St. John).
In addition, they rehearse - Bach and a few supplementary composers - up to two hours a day. They sing on radio and television; they make recordings; they give concerts in Leipzig, East Berlin, and other cities in the German Democratic Republic. They go on world tours that devour an estimated 2 1/2 years out of their total nine years at the Thomas boarding school. They study the Lutheran catechism, most of them. And they still manage to complete their regular Abitur, an academic diploma between high school and junior college level.
To say that these boys are overachievers doesn't begin to tell the story of the purity of their voices, their dedication to the strict discipline of Baroque counterpoint - or the exultation they find in singing.
''It's terrific fun living here and making music with the others!'' exclaimed 12th-grader Albrecht Sack spontaneously in an interview in the school dormitory. At the beginning he wasn't sure he would like it. But the life quickly grew on him, and now he is sorry it's drawing to a close.
To the background accompaniment of an erratic but exuberant tuba from somewhere in the bowels of the 1880s building, seventh-grader Norbert Raschke and sixth-grader Ernst Ruppke echoed Albrecht's enthusiasm. The two younger boys will sing in the boys' choir until their voices change; then, after they have settled into an adult baritone or tenor, they will graduate to the men's choir.
An alto rehearsal quickly proved that all the ''Thomaners'' share Albrecht's, Norbert's, and Ernst's zest. In a high-ceilinged, chandeliered room furnished with two pianos and busts and portraits of Bach and other cantors past, the boys read their way through a new piece.
''Now, my men, this is a very, very difficult cantata,'' warned Hans-Joachim Rotzsch, the shirt-sleeved present cantor and 15th successor to Bach. ''The Professor,'' as he is commonly known, had his men ''la-la'' a few pages. He would stop them now and then to go back over a section, tease one boy about his cello playing, or make a new choir member feel at home. He herded the boys along , voicing the alto part with them here, giving the full piano accompaniment there, sometimes playing only the solo line, sometimes giving only a decisive chord change.
Suddenly he let them go completely on their own, and the single line dissolved into a spread of guesses and giggles. There were whispers - but only about the music. And the next time through ''Forget not what good He hath wrought for you,'' the a cappella rang true and clear.
Clearly, Albrecht was right. It may be hard work. It may require total concentration. But above all, it's fun. It's exhilarating to explore and begin to conquer yet another invention of the crafty old master.
True to Bach, liturgical music remains the priority of the St. Thomas Choir. In recent decades the boys' repertoire has been widened to include secular music - and the choir gives regular concerts in the Leipzig Gewandhaus and irregular performances on such occasions as the East German Socialist Unity (Communist) Party Congress or the Kremlin festivities for the 25th anniversary of the founding of East Germany. Then, in another secular touch, it is the city government of Leipzig rather than the St. Thomas Church that appoints the cantor (as has been the case ever since the Reformation transformed Leipzig in 1539).
Nonetheless, it is the weekly church services and motets that form the character of the choir. Even in secular concerts half of the program always consists of Bach's religious music. And Rotzsch, himself a Lutheran, says the cantor must necessarily be a Christian.
The St. Thomas Church has thus succeeded in keeping the music of its world-famous choir as an integral part of worship rather than letting it become a mere vehicle of excellent musicianship. No tickets are sold for the weekly motets, which include hymn singing by the congregation as well as Bible reading and prayer by a minister. And even when tickets are sold for performances of Handel's ''Messiah'' at the St. Thomas Church, these are handled by the church itself and not by the state concert agency.
This religious integrity does exact one curious price in a city that normally prides itself on being one of the musical centers of the world. In a state that has far less religious than cultural awe, the religious nature of the motets is apparently taken by the state travel bureau as license to troop tourists in and out of the church during the music and worship.
That the state tolerates the religious identity of the St. Thomas Choir is attested by the number of official awards the group has earned: the Order of Service to the Fatherland in 1954 and 1972; the Art Prize of the City of Leipzig in 1972 and 1974; the 1971 Gold Medal of the 13th Worker's Festival; and the 1976 Art Medal of the Free German Youth.
A further indication of the special regard the Thomas School enjoys is its exemption from the military training that is otherwise compulsory in East German high schools. The exception for the Thomas School may not be based on religious grounds - Professor Rotzsch explains it instead in terms of the pupils' exceptional time pressure. This exception certainly does avoid one possible church-state conflict over the choir, however, since military training has been a bone of contention between the Lutheran Church and the state for several years.
In the future, if they follow the usual pattern of Thomaner alumni, Norbert, Ernst, and Albrecht will not become professional musicians. Albrecht wants to become a veterinarian; the two younger boys are undecided as yet.
If they do follow the usual pattern, however, they will all keep music as a hobby. And they will certainly keep the fond memories they already have of singing tours abroad - such as the concert in Valencia a couple of years ago, on the night of the attempted Spanish coup, or the monumental 50 pounds of spaghetti they once consumed in Italy.
The trio is convincing. It does sound like ''terrific fun'' to be a Thomaner.