The coming of Jesus Christ changed the nature of death.
More exactly, I believe that this happened at the point of his baptism by John; the time when Jesus became divine; when the divine spirit rested upon him and stayed with him.
From then; those who loved, trusted, had-faith-in, 'followed' Jesus (those who wanted to be resurrected and dwell in Heaven for eternity) would be resurrected.
So time is real, history is real; the nature of death is divided into before and after that moment. That moment introduced the new possibility after biological death; which was resurrection to eternal life.
Before Jesus, there was no resurrection. When Men died, the spirit was separated from the body. What then?
My understanding is that the body is what enables greater agency, greater freedom; our capacity to be an actor rather than acted-upon. A spirit without a body has a much lesser degree of agency; so when the body dies there is a loss of The Self.
We experience an analogous situation each time we sleep. Sleep itself represents two of the possibilities after death - when we live in the spirit.
Deep sleep is the loss of consciousness. We are alive but don't know it (or barely so); alive but unaware of anything. This is the nearest reality to the subjective perception of death as annihilation.
Genuine annihilation of an individual spirit is impossible since our primordial spirit had no beginning, is eternal, has no end - but self-awareness can be annihilated (which represents a return to our primordial state, before we became Children of God) - alive but unaware.
When this state of alive-but-unaware is pleasurable, blissful - then it is Nirvana; the state of being sought by Hindus and Buddhists. So I am suggesting that deep sleep is a temporary Nirvana.
Dreaming sleep is equivalent to Hades or Sheol; which are seen as conditions of 'delirious', or demented half-being; when men become witless ghosts or similar.
This is seen in the state of dreaming sleep insofar as we are in a passive state of being. Memory constantly slips away, our capacity for agency is feeble so that we 'go along with' whatever is happening.
Dreaming sleep is an experience of passivity, loss of reason and purpose. It is a vision of spirit life without incarnation.
I suggest that these states - Nirvana and Hades, corresponding to deep and dreaming sleep - were the possibilities of spirit life before Jesus.
A further possibility was reincarnation. The spirit could be re-housed in a new body.
Since the body, and its specific nature, affects the spirit - this meant the reincarnated spirit, reborn and leading another life, was 'a different person' - not the same person repeated.
An analogy would be a relative who shares a certain fundamental similarity, the same flavour, deep character - "He's Just Like his uncle John...".
After Jesus a further possibility was introduced, in addition to 'Christian resurrection' - and this was Paradise.
Paradise takes various forms - Valhalla, or the Muslim Paradise. Implicitly, Paradise is a state in which our-selves are retained and our agency; so paradise is a kind of resurrection.
But Paradise is not a resurrection to the presence of God and the participation in the work of creation that is Heaven. It is a place where one's favourite activities become possible, in principle eternally (and subject to the limits of that aspiration, and the constraints of mutual existence).
Paradise (in its variants) is, indeed, pretty much the lower or 'Telestial' Heaven as described in Mormon theology. It is pleasurable and enjoyable, but in Paradise men are not qualitatively different from how they are in this mortal life - there is no ascent to a higher, more conscious and creative and loving, form of life.
In sum; Paradise is essentially uncreative, passive ('contemplative', appreciating, consuming) a reversion to childhood or adolescence; to Original Participation. And I believe it is possible that some people in Heaven are actually experiencing Paradise - e.g. those who are resurrected as (in their essence) children, but who live (as children) with their families who include those who are participating with God in the work of creation.
What about Hell? Well some will choose that, on the basis of how they choose in mortal life - maybe even a large majority of people in the modern West.
These are self-excluded from heaven, and self-excluded from resurrection; Hell is the exclusion of Love.
Such remain spirits in the condition of Sheol, but isolated by the perspective and priorities of those who choose Hell.
Their state seems terrible to me; and is based upon a primary (pride-full) dishonesty of denying that they are God's children living in God's creation... but Hell is what they get, having rejected all the above.
So, Jesus brought Hell, as well as resurrection in Heaven - because it is deliberate, conscious rejection of the world of God, Good, Creation and Love that makes Hell hellish.
Note added: Resurrection is the single most astonishing, incredible, mysterious thing about Christianity. That is my point. What that means is that resurrection is Not something that can be 'explained' in common-sensical, ordinary, easily intelligible, procedural terms as if it was a chemical manufacturing process. It is incredible. I am not At All surprised if people don't believe it. Nonetheless, resurrection is something near the core of what Jesus taught (and did). I think resurrection is probably a much more important fact of Christianity than commonly regarded. We should work from that, rather than try to make the incredibility go away.