This tradition began with the importation of the Byzantine Empire's religious music when the Kievan Rus' converted to Orthodoxy in 988.
[1] When Prince Vladimir of Kiev converted to Orthodoxy, Mount Athos was the musical center of the Orthodox world.
Monks from all over Eastern Europe and Byzantium traveled to Mount Athos for musical training and to learn the ways of Orthodox chant.
[2] At Mount Athos, Russian monks learned the Byzantine neumatic notation for chant, which they readily adopted and brought back with them to Russia.
Additionally, regional variants and neumes became part of the established tradition in those areas, making it nigh impossible for singers to sight-read any chant from paper.
Eventually, during the Polish Renaissance, the Kievan Orthodox Church fully adopted the polyphonic styles popular at the time.