The promise of Jesus in the Fourth Gospel is that those who believe in him will have the reward of life eternal/ life everlasting.
As Jesus explains to Nicodemus; this is to be attained only after mortal life: via biological death and being born again.
Life eternal/ everlasting is the state of resurrection; and its consequence explained in terms of the various events of the gospel when Jesus contrasts the satisfactions of this mortal life - wine, water, bread, meat, sight - with the great, qualitative enhancement that these mortal experiences will have in the life to come.
How can we understand this?
The answer is that the resurrected body is what transforms worldly experience into Paradise. The resurrected body is eternal, everlasting, indestructible - and has divine powers of both perceiving and thinking.
(Because the incarnate body and the soul are indivisible, therefore transformation of the body is itself a transformation of the Man.)
The resurrected Man has, in effect, 'extra senses' unknown to mortal Men, and creative powers of imaginative and intuitive thinking.
(These we may experience briefly, in the context of our constantly-changing mortal state - but mortality is primarily for learning, rather than doing.)
Thus the promise of resurrection is itself the cause of the astonishing enhancements in the quality of living that Jesus promises. It is because Jesus brought resurrection that he also brought the possibility of Heaven.
Note: This was clarified for me by the discussion of William Blake's Marriage of Heaven and Hell on pp 194-5 of Fearful Symmetry by Northrop Frye.