Isaac’s six-part Missa De Apostolis
is based on a selection of Gregorian chants taken from the repertoire of the Feast of the Apostles. It will immediately be noticed that the music follows a number of the unusual and conservative liturgical procedures current in Germany and Austria around 1500, when this music was written. In particular a composer was expected to set the Mass in alternation between chant and polyphony, section by section and in every movement. This turns the Kyrie, for example, into a full invocation in nine separate sections. The chant duly carries straight through from the monophonic verses into the polyphony, appearing in all the different voice-parts at different times as a decorated cantus firmus. This process unfortunately breaks down in the Gloria where it proved impossible to rediscover the chant on which Isaac had based his music, and another one in the same mode had to be substituted. In addition the Credo was not set because it was not then customary to include it as part of the polyphonic Ordinary. In fact Isaac left a large body of separate polyphonic Credo settings, which presumably were used in contemporary services, but none was found to fit this particular chant and vocal scoring. Although his polyphonic sections are constantly being interrupted by statements of unadorned chant, Isaac managed to set his texts on the grandest scale. This unhurried breadth of effect is one of his most valuable artistic achievements.
from notes by Peter Phillips © 1991