Apparently Justice Department official Robert Mardian -- later of Watergate fame -- circulated a memo saying that Leonard Bernstein was being assisted in creating the piece by "show business personalities in New York and Los Angeles," and that "'Jesuit friends of the composer' are serving as advisors" -- among these, war protester Father Daniel Berrigan. And this, Mardian thought, might presage a clever anti-American PR coup: "One could surmise," wrote Mardian, that "Bernstein has requested Father Berrigan compose words for the 'Mass' in Latin and it would follow an anti-war theme." Thus Bernstein was suspected of trying to get "high-ranking Government officials" who "probably are not conversant in Latin" to applaud anti-war sentiments at the gala premiere. (So far as we can tell, the Latin bits in Mass are from the Catholic liturgy.)
Link (here) to the Village Voice.