One of the deeper mysteries in the mystery-packed life of P.D.Q. Bach surrounds the circumstances of his excommunication from the Roman Catholic Church. In his "Definitive Biography" of the last and least of Johann Sebastian Bach's sons, which has just become available in paperback (Random House, $5.95), Profesor Peter Schickele tiptoes around the subject i a rather gingerly style, with occasional offhand allusions - not because he finds the subject distasteful (he seems to finds nothing distasteful) but because when the book was written he knew practically nothing about it.

The few facts than can be gleaned from Schickele's "definitive" study may be summarized as follows: P.D.Q. become a Catholic sometime before 1777 with the attention of entering a cloistered religious order. This attention was thwarted when it was discovered that he planned to spend his life in a cloister of nuns, not of monks, but he remained a not very faithful member of the church until his excommunication in 1787. He served for a time as Temporary Substitute Assistant Organist and janitor in the Cathedral of St. Jezebel in Hymarket, and within two years (apparently by blackmailing in the bishop) rose to the rank of Organist of the Chapel. One must assume that this was not a Catholic cathedral, since none were allowed in England at that time and the name of Jezebel does not appear on that church's calendar of saints. But this defection does not seem to have bothered authorities in Rome, if they knew of it at all, in fact, they may have felt relieved. His religious work - all of which were deliberately lost - include the "Mass in the Allah Mode"(influenced theologically as well as musically by a visit to the Near East) and the "Half-Nelson Mass" and "Missa Hilarious," both apparently written after his excommunication. The notoriously tedious "Boston Mass," it seems, was not written by P.D.Q Bach, as Albrechtsberger believed.

In the absence of any of the composer's religious music, it has been hard to understand why he was excommunicated; his hobbies included lust, gluttony, sloth and habitual drunkeness, but these are regarded by the church as a human weaknesses, not as the kind of theological offense against which excommunication is invoke. Now that P.D.Q. Bach's "Missa Hilarious" has been discovered and performed, the mistery is cleared up and a new one substituted. On a new recording, "Potrait of P.D.Q. Bach" (Vanguard VSD 79399), the music proves to be the most serious distortion of a scared Latin text since the Council of Trent. One hearing makes it understandable that the church terminated the composer's membership but raises the question of why he was not burnt at the stake. Schickele notes that "two strangers with Italian accents" kept the composer under surveillance off and on for several years after the perfomance of this music, but apparently no further action was taken.

P.D.Q. an announces his sacrilegious intentions from the very beginning, setting the first "Kyries in Pig Latin ("Yrieky) and inserting exotic episodes into the "Christe Eleison." One of these is Swiss ("Christe yo-deleison") and another is Spanish ("Christe ole," bringing in castanets and a stirring pasodoble rhythm). By the time he picks up the "Kryle" again in a sturring chorus ("K-K-K-Kyrie"), the listener's mind is too numb with shock for further meaningful protest.

Other offenses are too numerous to mention, but we can hardly drop the subject without citing the use of the "Hare Krishna" melody for the "Sanctus," the blatant bid for popularity in the "Gloria" ("I've just met a girl named Gloria"), the gross theological misreading whereby the Holy Ghost is referred to as "Spiritum Inner Sanctum" or the forced and inappropriate jollity of the "Ho-Ho-Hosannah." In the "Credo," the composer allows musical considerations to override those of doctrine ("Credo-re-mi-fa-sol-la") and he guilty of overelaboration at the end of this section, inserting "Ah, men; ah, women; ah, wilderness; ah, nuts; ah-choo" where a simple "Amen" is all that is required. Compared to these offenses, his theft of the melody from Handel's "Halleljuah" chorus for this "Agnus Dei" is a mere pecadillo.

In fairness, I must add that this is probably the funniest piece of Music I have ever heard - at least the funniest since the end of the Hoffnung Interplanetary Music Festivals. But that's not what we want in religious music, is it?

The remainder of the record is filled with two more works of P.D.Q. Bach and Schickele's own "Eine Kleine Nichtmusic," in which it is impossible to say which is worse - the parts stolen from Mozart's music of (almost) the same name or the parts stolen from everywhere else.

P.D.Q.'s "Encho Sonata for Two Unfriendly Groups of Instruments" is somewhat better performed than in its only previous recording, which doesn't necessarily mean that it is very well performed. By far the finest music on the record (though there are some verbal infelicities) is the concluding "Consort of Choral Christmas Carols," which includes the lovely "O Little Town of Hacken-sack," the spirited "Good King Kong Looked Out" and the convical if fuddled "Throw the Yule Log On, Uncle John." It is a pity that the all-important comma in this finaltext was not more carefully observed in the music.