When Heinrich Isaac took up the prestigious post as Court Composer to the Holy Roman Emperor Maximilian I around 1497, the local style and specific liturgy of the Imperial Court demanded a new type of composition from him: the Austro-German Alternatim Mass. His previous musical employment in the brilliant (but worldly) environs of Medicean Florence called for stylish mass settings based upon popular French love songs. The Imperial custom, on the other hand, required that the mass celebration on high feast days be adorned with polyphonic music based upon the appropriate sacred chant of the Catholic liturgy. In both styles, Isaac left numerous mass settings of the highest compositional quality and wit. The three mass cycles surviving under the title Missa De apostolis (Mass for the feast of an Apostle) apparently were intended for these more solemn Imperial rites.

The Rite of the Diocese of Passau, observed by the Imperial chapel, gives a specific series of chants for the feast of an Apostle, and these chants provided the compositional basis for polyphonic elaboration in Isaac's three masses. Each cycle—one for four voices, one for five, and one for six—contains four movements: Kyrie, Gloria, Sanctus, and Agnus Dei; this court did not usually sing the Credo in polyphony, though Isaac composed a large number of isolated Credo settings which might have been inserted. Local custom also called for only certain subsections of the movements to be set; the alternate verses would either be sung by the choir in unadorned plainchant, or might be spoken by the priest during an organ improvisation upon the chant.

Though thus comprising a series of fragmentary musical settings, Isaac's three masses on the "De apostolis" chant show the riches of his musical imagination. His most common technique is known as Cantus fractus, a practice which differentiates the chant-bearing voice by means of longer note values. The well-known chant melody is thus highlighted in the texture. But the composer passes this chant freely between voices from one movement to the next, once (in the six-voiced Sanctus) even dividing phrases of the chant between voices midstream. Many movements (Kyries especially) open with imitation in several voices upon the chant's incipit, building intensity and rhythmic drive gradually to the end. The Apostles' Masses also contain quite a number of skillful canonic passages, including canonic repetition of the chant melody between two (or three) voices, close canonic imitation of a contrapuntal voice, one instance of a double canon (the five-voiced Quoniam, which contains both types at the same time), and two crab canons, wherein a pair of voices simultaneously sings the chant melody forwards and backwards.

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