The ruling class of any society decides what the masses believe. This is the purpose of public education and mass media. Media - not just "news", but much of entertainment as well. Public indoctrination through entertainment has been going on a very long time, before television and movies, there was required attendance at theater productions tailored to instill certain values/beliefs in the general public.
Religious literature is not exempt. Though there is undoubtedly truth in some common religious texts, there is also evidence they are not exempt from ruling class tampering. "God created your government/laws/ruler" / "obey the King" type messages are found in various parts of the bible for example.
The question of the OP was who or what determines truth. The ruling classes are generally not concerned with it as far as the general public is concerned - they are much more concerned with occulting (hiding) important information that would (if known), upset the balance of power out of their favor. That of course doesn't mean they lie about everything, but they do lie about a great many important things.