You write about the Latin mass. That makes your post seem like it is ignorant of the context of this HN post. The described manuscript is a Greek one, and Eastern Christianity did not use the Latin mass. Now, Greek liturgy is in a fossilized form of Ancient Greek (Koine, or Byzantine with some Atticisms) and certainly poses some problems of understanding for modern parishioners who speak Modern Greek, but less so than Latin for Western Christians. Moreover, popular literacy in the Byzantine world remained higher than in Western Europe after the fall of the Roman Empire.