Let us finish the story during the time period that you speak of. The far majority of people were illiterate, elementary schools were not a thing until later, and until sometime after the printing press became common, Bibles were not readily available for anyone, even the literate.
Indeed. Let us also consider:
It is not the case that the population was kept ignorant of the Bible, in that those who were even somewhat educated would be able to understand the lections read at Mass and the Divine Office, and speaking of the latter, it has generally always been available in cathedrals, the problem with what Robert Taft SJ, memory eternal, refers to as the “devotionalization” of the Divine Office in his excellent history The Liturgy of the Hours East and West being largely a parish problem.
It must be stressed that in the Medieval period, furthermore, Latin was still widely understood, as it remained the Lingua Franca of the age, except in England, where it was used alongside Norman French, hence the origin of the term Lingua Franca, and in Southeastern Europe and Asia Minor and the former Byzantine Empire, where Greek was the Lingua Franca, and the Slavic lands where Church Slavonic served as the Lingua Franca.
And in Croatia and Dalmatia, the Roman Catholic Church celebrated the Galgolithic Mass, which is the Roman Mass in a dialect of Church Slavonic written using Galgolithic Latin rather than Cyrillic characters, into the 1970s, and even today on occasion. And likewise part of the raison d’etre for the Byzantine Catholic Churches and other Sui Juris Eastern Catholic Churches is comprehensibility, as the Roman church sought to ensure that the Christian people in lands which came under the rule of Roman Catholic monarchs retained worship in a form comprehensible to them, and later on went so far as to prohibit the imposition of the Latin Rite on communities accustomed to worship in one of the Eastern liturgical rites.
And we must also consider that Latin, even the Classical Latin of the Mass, which is shared with the Vetus Latina Bible originally translated along with the liturgical texts in the second century at the direction of Pope St. Victor for the benefit of those citizens of the Western Roman Empire who did not speak Greek. In the city of Rome, these were predominantly middle and working class citizens and also slaves, who did not benefit from access to the Rhaetor, the Roman secondary school, or even the Grammaticus, let alone education in the Greek tongue, which was something that was widespread only because the upper classes desired it and the merchants and many civil servants and military officers having anything to do with the Eastern empire required it). This helped greatly in Rome but even more so in the provinces of the Western Empire, where outside of the Province, as the Romans liked to refer to Southern Gaul (hence the name Provence for the beautiful Mediterranean coast of France), knowledge of Greek became less and less widespread, so that one might well find it scarce even among the upper classes in a place such as London or Trier, for Latin was the language “understanded by the people” to borrow a phrase from Cranmer.
And as
@narnia59 noted, the Roman church, when the Vetus Latina started to become harder to understand with the emergence of the Vulgar Latin dialects which in turn developed into the Romance Languages, the Roman church responded by commissioning St. Jerome to do a new translation, which became the Vulgate, and this work was an extremely important contribution to all of Christendom, in that, like the Syriac Peshitta and the Ge’ez language scriptures of Ethiopia, and the Septuagint, this directly translated the books of the Old Testament from the Hebrew and Aramaic in which they were written.
These translations help provide us with multiple sources of Old Testament material, which is invaluable when we consider the tragic loss of not only Origen’s Hexapla but several of the Old Testament versions it contained (such as Symmachus and Aquila).
In addition, even in the periods of peak illiteracy during the Middle Ages, the Roman Catholic Church worked around the lack of literacy through vernacular preaching, and indeed the Dominicans were originally established specifically for this purpose, to preach to the Albigensians to persuade them to convert back to Christianity from Gnosticism. St. Dominic is one of my favorite saints for this reason. And the other Mendicant orders which developed concurrently with or subsequent to the Order of Preachers (the former title of the Dominicans), particularly the Franciscans, but also the Carmelites, Servites, Minims, and the ransoming orders*, the Mercedarians and Trinitarians, also developed a reputation for excellent preaching, as did the various groups of Canons Regular, such as the Augustinians and Norbertines. The Norbertines, like the Dominicans and Carmelites, also developed a standardized liturgy for use wherever they were assigned, and in many respects, the beautiful Dominican, Carmelite and Norbertine uses of the Roman liturgical rite paved the way for the later introduction of the standardized Tridentine mass and divine office in the 16th century. Likewise, the Tridentine mass and the standardization it provided helped facilitate the implementation of vernacular liturgies following Vatican II (since much of the Novus Ordo Missae is directly adopted from the Tridentine liturgy, for example, the Liturgy of the Word, and Eucharistic Prayer no. 1, which is based on the Tridentine form of the ancient anaphora known as the Roman Canon.**
*These two orders were founded for the special purpose of raising funds to ransom the large numbers of Christians abducted by Islamic terrorist pirates from North Africa, who somewhat like the Somali pirates of recent years, held them for ransom (unless they converted to Islam). These two orders added an additional vow to the standard vows of poverty, chastity and obedience which characterize the other mendicant orders, in that Trinitarian and Mercedarian Friars additionally agree to substitute themselves for, and take the place of, any hostage who is at risk of being converted to Islam. The importance of the work of these orders is one of the many unsung and underappreciated triumphs of the Roman Catholic Church, in that when the Trinitarians and Mercedarians were formed, at the end of the Middle Ages, Islamic pirates, protected by the rulers of North Africa, did not limit themselves to piracy at sea, but rather frequently raided the coastal highways of Southern Europe, so that even ordinary people walking from Naples to Rome on the old Roman highways were at risk of abduction.
**The beauty of the Roman Canon as an anaphora was not lost even on the Eastern Orthodox, for the Russian Old Believers made use of a Sluzhbenik, or Liturgikon, basically a priest’s service book, that contained in addition to the more well known Divine Liturgies of St. Mark and St. James, the Divine Liturgy of St. Peter, which consisted of the Byzantine synaxis, or Liturgy of the Word, with the Roman Canon.