Just below the surface; it is a puzzle that Jesus promises eternal life; when life is, apparently, already eternal. Souls do not die, and both Jews and pagans of Jesus's time (or leading up to that era) assumed that souls continued in a kind of underworld. What, then was so special about everlasting life?
What is being offered by Jesus is that we our-selves will live eternally - and that this includes resurrection, a permanent restoration of the body. Because - for both Jews and pagans - what of us that persisted in the underworld after we have died was no longer our-self.
The implication is that for we our-selves to live forever requires that bodies (our bodies) must also live forever.
This can't happen with our mortal bodies; they never were suitable for eternal use - from conception and birth there are problems, and through life these accumulate, and we die. So a permanent body must be another one that is (in some sense) the same as the mortal one, but not the same one.
This isn't really any mystery or paradox - because each person's identity (our identity) is based upon the linear continuity of our-selves through time. So resurrection is understandable as the continuity of our selves, souls, from eternity - going through a phase of mortal incarnation which is (for some reason that we don't understand) developmentally-necessary for the development of a resurrected body.
So, these are the necessary developmental stages of a Man. We must go-through these phases - spirit, mortal body, immortal body - if we want to become eternal selves...
But the essence of the necessity of Jesus's life (i.e. why Jesus was needed) is that the 'final' stage - of transition from mortal to immortal body - is one that requires our conscious assent; unlike earleir stages, resurrection does not 'just happen'.
We must want it, and we must want it in a particular way that includes wanting the consequences of it; which means that resurrected life eternal is not just about our-personal-selves living forever; but about the fact that to have this is to become gods - and participating in the 'ongoing work of creation'.
The two go together - resurrection and participation in creation; which is a clue to the fact that resurrection is itself an aspect of ongoing creation.
