Luther was not the first to translate into the vernacular though --
The idea that the mediaeval Roman Catholic Church was consistently and universally opposed to translating the Bible is something of a myth. It's true that there were cases where specific Bible translations were forbidden, but it was never a blanket prohibition. None of the people often quoted alongside Luther- Wycliffe, Tyndale and Hus - were condemned for translating the Bible as such, but for more general crimes of heresy and opposition to the Church hierarchy. Their books were proscribed along with their other teachings.
An English translation of the Bible written in about 900 CE, which was not prohibited in any way. (The third line down, with the capital letter, says Ða cƿæð se hælend: fæder, forgyf him forþam hig nyton hƿæt hig doð - or in modern English, "Then said the Saviour, "Father, forgive them, for they know not what they do".)
First off, it's important to remember that the idea of the bible existing as a self-contained whole is something that only started to take shape late in Antiquity and it only really became normal in the Middle Ages. In the earliest days of the Jesus Movement, congregations would be conducted with only readings of the Letters of Paul as well as relevant passages of the Septuagint, which was the authoritative translation of the Old Testament that had been formulated centuries earlier by members of the Jewish community in Alexandria. Around the turn of the 1st Century CE, this practice was supplemented with readings from the Gospels as well as other works which never made it into the Christian canon, e.g. The Shepherd of Hermas and various apocalypses.
Secondly, Latin had largely replaced Greek as the language of the common Christian of the Roman Empire in many regions. This meant that all the texts of the New and Old Testaments had to be put into accessible (vulgar) Latin. Hence the term "Vulgate." This task mostly fell to Jerome, who was universally regarded as a brilliant linguist and an incorrigible asshole in his own day. Although it was perceived as eccentric and tedious, Jerome was adamant about consulting Hebrew translations to corroborate his work on the Septuagint, and in some cases he referred to Aramaic as well. (The Masoretic text that we now think of as as the Jewish Tanakh wasn't yet standardized, though it's mostly faithful to Jewish texts dating back to the Dead Sea Scrolls.)
The point is, coming to some semblance of agreement about how to understand "What is scripture?" was such a burdensome task that, once the job was done reasonable well, nobody wanted to deal with it anymore. The list was made and the text was fixed by a bona fide saint. Ersatz translations were popular for the sake of evangelizing far-flung populations of the Low Middle Ages (that is, converting pagans) but there was still a general attitude that everyone should avoid straying from Jerome's Vulgate as much as possible. If it ain't broke, don't fix it.
So, come the High Middle Ages, there was a general tolerance for the bible to be rendered into the vernacular but there was a staunch consternation about treating the translated bible as "the word of God." The concern wasn't that Latin was a more holy or refined language than Old French or Occitan or Old High German or whatever else--Latin was originally chosen precisely because it was more accessible than Greek--but that no armchair linguist could do for Latin what Jerome did Greek and Hebrew. The bible was too important to be left to the hands of amateurs, and the professionals felt they had better things to do than translate since they themselves didn't need a translation.
The same thing with the KJV versus NIB today