Some things are more meaningful than others. Some purposes are more important than others. Some ethical considerations outweigh others. Some aspects of your identity are core to who you are, whereas others are incidental.
But often it is hard to compare. What is the most important thing to do? Which choice should you make in an ethical dilemma of conflicting principles? Which parts of your identity are you least willing to let go of in difficult, changing circumstances?
Eternalism hijacks two accurate observations, that some things are clearly more meaningful than others, and that much suffering can come when a comparison is not clear. It promises an “ultimate” standard that would resolve all such questions.
“Ultimate” is also used to mean “greatest and most important.” Greatest how? Most important why? The word is extremely vague, but sounds impressive, like whoever says it must know what they are talking about. That makes it ideal as a weasel word, like “really .” Eternalism typically uses “ultimate meaning” non-specifically, as synonymous with “real meaning” or extra-special fancy meaning. That means “do whatever we say and don’t ask questions.”
Nihilism , on the other hand, often takes the word more seriously, asks what “ultimate meaning” would actually mean, and comes up with some specific answers. It observes that there isn’t any meaning like that, decides this is somehow awful, and descends into depression.