I can't tell you very much about this very prolific South Netherlands composer's early life neither his birthday which could be any time during the five-year period 1450-55 nor his birthplace are known. He was a layman who and appears not to have attended university what he was was a very accomplished composer some of whose music was being published in Innsbruck by the mid 1470s. He worked variously for Duke Sigismund of Austria, the baptistry of S Giovanni in Florence, Florence Cathedral the Servite friary of SS Annunziata and Lorenzo de Medici. His work for the Medicis seems to have included acting as teacher to Piero and Giovanni de Medici (Giovanni de Medici became Pope Leo X in 1513 ) both of whom became his patrons. The changing political circumstances in Italy led him to seek employment elsewhere and he wound up working for Maxilmillian I for a while. However his Medici patrons did not forget him and arranged for him to become provost of the chapter of Florence Cathedral. He died in Florence in 1517. His music is both beautiful and interesting but it's as a teacher that he was most influential – the list of his pupils includes such names to conjure with as Adam Rener, Balthasar Resinarius, Petrus Tritonius, and last but by no means least Ludwig Senfl.
His Missa De Apostolis was written for the Viennese court and is based on on a selection of Gregorian chants taken from the repertoire of the Feast of the Apostles (which is why it doesn't include a Credo as Viennese practice was not to set the Credo). It has a six-voice texture which was very unusual for the period and is an alternatim setting, it's a very terse piece whose constant alternation between chant and polyphony means that many of the polyphonic sections consist only of a single phrase. Isaac solved the musical problems that this brevity presented by making heavy – and spectacular, use of both of sonority and dissonant cadential writing. It's almost English in its sound. Enjoy :-)