Kierkegaard defines the self as a relation that relates itself to itself. That means that who I am depends on the stand I take on being a self.. see stance!
We’ll see, though, that almost everyone adopts the nihilistic stance at times, without noticing. When the complete stance is unknown, nihilism seems like the only possible defense against the harmful lies of eternalism. (Just as eternalism seems like the only possible salvation from the harmful lies of nihilism.)