It was Jesus Christ who made Heaven possible: there was no Heaven before Christ.
That was the main thing He did.
Jesus brought Men resurrected life eternal; and it is resurrection that makes Men wholly and permanently good: that is, wholly and permanently aligned-with, and in-harmony-with, God's creation and its purposes.
And Heaven is the wholly-good mode of existence.
In pre-mortal life, when we were spirits, we did not dwell in such a state of perfection. In that childhood of the spirit; those were good who obeyed God - and this obedience was unconscious and spontaneous.
Pre-mortal life is analogous to the goodness of a good young child; whose goodness consist in obedience to good parents.
But while pre-mortal spirit life is good - it is not wholly-good. There is, to varying degrees, evil in all pre-mortal Mens' spirits - natural evil, from basic-selves; from their original nature as Beings who existed out-with divine creation; and from desires that are dissonant-with, and perhaps opposed-to, creation.
Yet, while pre-mortal spirits are living in loving obedience to God; this evil is prevented expression.
However, some pre-mortal spirits do not love God, disobey God, and leave the divine presence. These are the fallen angels or demons.
Demons can be understood as pre-mortal spirits who initially dwell in the Heavenly state; but (sooner or later) reach a point that cannot obey God, and cannot live (even a unconsciously and passively) in obedience to God.
They are (from their basic selves, in their original nature) so evil that it (sooner or later, perhaps quickly, or perhaps after some development of the spirit) breaks-through and demands expression. They do not love God enough to obey - probably, some are incapable of love altogether.
This is analogous to a nasty, wicked young child of good parents; the wicked child desires to do evil things, and rejects the good instructions of his parents by refusing obedience. Or else the child does not love his parents and sees no reason to obey them.
Or, in extreme situations; the child is incapable of, or excluding of, love; and knows of no reason why he should not do exactly what he wishes - if that is possible.
We can imagine a demon as being like a child who (from some mixture of innate nature, and learning) becomes so evil that he runs away from the loving environment of his parental home, and perhaps joins a gang of like-minded thieves or murderers ... whatever enables him to enact his wicked desires.
(This running-away is thus analogous to the fall of angels, to the emergence of demons.)
Such a child's rejection of parental goodness does not require a conscious awareness of what he is rejecting. He makes his choice on the basis of what he most wants to do, and seeks to escape an environment that prevents him. He is 'in thrall' to sin.
In sum; the contrast between the Heavenly life of unfallen pre-mortal spirits, and the Heaven of post-mortal resurrected Men; is that in pre-mortal life Men still have (to varying degrees) a disposition to evil.
The spirits are only partly-good by nature, but behave with goodness because of (unconscious, spontaneous) obedience to God.
Whereas, after resurrection, Men in Heaven have left-behind all their evil nature, all impulses towards sin (i.e. all inclinations to depart from the purpose and harmony of divine creation).
Resurrected Men have been transformed to wholly-good inhabitants of that state of eternally-good heaven. This transformation must be chosen, must be assented-to - or else it cannot happen.
Therefore the state of Heaven is one of wholly-good beings who are motivated only to do good; whereas the pre-mortal state is of partly-evil beings who (for so long as they remain in this state) behave with perfect goodness - yet not from inner motivation, but from obedience to God.
There are therefore several choices here, with varying degrees of consciousness.
The choice of a pre-mortal spirit to become a demon is largely un-conscious, and rooted in nature and desire.
The choice of a pre-mortal spirit to move-on to mortal incarnation is more-conscious choice to embark on spiritual development towards greater consciousness; with the goal of making a permanent choice for or against resurrection.
In other words; pre-mortal spirits who desire to become more like God (analogously to a child desiring to become more like his parents) want to grow-up; and enter mortal incarnated life; which is somewhat like the phase of adolescence between child- and adult-hood.
(Presumably some pre-mortal spirits do not want to grow-up, do not incarnate, and remain in that Heavenly state.)
And at the end of mortal incarnate life; comes the possibility of making a conscious permanent choice to become wholly-good (wholly God-aligned) and undergo the transformation of resurrection...
Or else of rejecting this.
(I believe that - in principle, in some times and places - there are several possibilities for those who reject resurrection; but in this era, in The West, it seems that more-and-more of those who reject Heaven will instead choose Hell. At least, that is what they say, and what their behaviour indicates.)
The sequence is therefore a process of development, a maturation, a growing-up - through a series of choices; and the main change throughout (if the sequence goes according to God's wishes) is on of increasing consciousness of those choices.
If God just wanted good-behaviour from us; then the spontaneous, natural, passive obedience of pre-mortal spirit life would suffice.
But God wants more!
God desires that we grow-up to become our-selves more god-like; and part of this is making a conscious choice to align ourselves with divine creation; and to do so permanently by means of resurrection.
Only after resurrection can we freely participate-in and contribute-to the work of creation; because then we will be wholly-good in terms of our alignment; such that all our creative activities will naturally and harmoniously contribute-to the eternal development of divine creation.