Adémar of Chabannes (989-1034) was an 11th-century monk, a historian, who wrote the first annals that had been compiled in Aquitaine since late antiquity, as well as a musical composer and a successful literary forger. He embraced the developing tale that Saint Martial, the 3rd century bishop who christianized the Limoges district, had actually lived centuries earlier, and was in fact one of the original apostles. And he supplemented the less than scanty documentation for the alleged 'apostolicity' of Martial, first with a forged Life of Martial, as if composed by Martial's successor, Aurelian. To effect this claim, he composed an "Apostolic Mass" that still exists in Adémar's own hand (Paris. Bibliotheque Nationale. MS Latin 909), making it the earliest autograph musical composition that has survived. The local bishop and abbot seem to have cooperated in the project and the mass was first sung on Sunday, 3rd August 1029.
Unfortunately for Adémar, the liturgy was disrupted by a travelling monk, Benedict of Chiusa, who denounced the improved Vita of Martial, as a provincial forgery and the new liturgy as offensive to God. The word spread, and the proimising young monk was disgraced. Adémar's reaction was to build forgery upon forgery, inventing a Council of 1031 that confirmed the 'apostolic' status of Martial, even a forged papal letter. The reality of this pathological tissue of forgeries was only unravelled in the 1920s, by a historian, Louis Saltet. Mainstream Catholic historians ignored Saltet's revelations until the 1990s.
In the long run, Adémar was successful. By the late 11th century Martial was indeed venerated in Aquitaine as an apostle, though his legend was doubted elsewhere. In a very direct way, Ademar's Mass shows the power of liturgy to affect worship.
Adémar composed his musical Mass and office largely from the standard "Gregorian" music for St. Martial, as well as texts and music for Apostolic feasts, but he also added some of his own compositions, especially in the tropes (extended musical items added to existing liturgical texts). The composition has been recorded by the New York Ensemble for Early Music.