Something similar happened in Islam: the sacred book and the liturgy are in Arabic, and if you want to understand them, you need to understand the language they are written in. Arabic is, of course, still a living language, but spoken and written today in widely varying dialects, which are not the same dialect as that of the Koran.
>not the same dialect as that of the Koran
For example, isn’t it true that Arabic generally does not have written vowells, but the Arabic of the Koran does?
Arabic can be written with or without vowel marks any time. Ordinary text gets along fine without them. They’re written explicitly in texts for young children learning to read, in Arabic grammar instruction books. They’re used throughout the Qur’an to avoid any possible misreading. Even in ordinary text, where the context of a given word allows for more than one reading, ad hoc vowel marks may be used to remove ambiguity.
The Qur’an is said to have originally been in seven different ancient Arabic dialects, and the text without vowels allowed for variant readings. Seven of these possible readings became established early on, though for many centuries, only one of them has been used worldwide and the others completely forgotten (except for one which survived in Northwest Africa). The vowels printed with the text have to follow one of the seven readings consistently. But you’re right, none of these seven corresponds to any of the vernacular forms of Arabic spoken today.