[Thought experiment] If alignments are objective how do we know what they represent?

[Saint-Just]

In most fantasy settings, including D&D, one answer to this question is 'so that when I die I don't spend an extremely lengthy period of time suffering horribly.'

I know it's a tangent, but I am having trouble remembering offhand one setting which has not started as an RPG where it is a known reality. I can remember maybe two which has dominant religions teaching that but no proof. In general fantasy as I know it tend to have unknown afterlives (unless they go for reincarnation) and even religions that teach either about unknowable afterlives or about something different from heaven-and-hell.