[Earw0rm]

"believe" and "true" as you're using them are relatively modern, secular concepts, the idea that anything can be tested to destruction and deemed true or false, and the idea that stories are superimposed upon the world rather than being inherently part of it.

If as a pre-modern person, you don't believe in a God or Gods, what fills the void? Nowadays the boundary of our knowledge stretches far beyond day to day life in many respects, but if you don't know about a round Earth, or the solar system, or how weather systems work, how else do you explain phenomena that have a material impact on your day to day life as a subsistence farmer?

Pre Christianity, there wasn't really God's law and Man's law, there was just law. Even today, if you ask a highly educated religious person about their faith, you'll often find a concept of truth in some ways broader than the secular, material variety.