All religious people need to have at least some understanding of death and what it is. In fact, everyone needs this since we all die but those who are serious about the spiritual life should do so in particular. The basic thing to say is that it is the shedding of the physical body and the release of the soul into the spiritual world but then you have to ask what is the soul and what is the spiritual world?
In the post mortem sense the soul is you, your mind and character which, to begin with at least, remain unchanged. The body is gone and one must assume that all conditions, mental as well as physical, that were linked to the body are removed. Does this mean that the blind can see and the senile regain their faculties? I would doubt there is a sudden change as that might be too shocking but there may be a gradual lifting of the darkness, both visual and psychological. Then the person stands forth as he is. But where is he? That depends on the person. In the spiritual world outer reflects inner so your environment after death will reflect your spiritual state. For some that might mean a plane of dimness and emptiness, for others there will be light both in the sense of illumination and the feeling of no longer being pulled down and trapped by the heaviness of matter.
For all, though, the death of the physical body will be followed, sooner or later, by the stripping of self. First of all, the physical body has been shed but there remain many other layers of falseness and artificiality that we have built up and surrounded ourselves with as a shield and defence and wall against life while in the world. We all have an image of ourself with which we identify and which we project outwards and this must be dismantled completely before we are ready to move on to more celestial regions.
So, the first thing to be done after death if we wish to ascend is purification. This corresponds to purgatory in the Christian tradition. Only the pure can enter a world of purity. There can be no darkness in heaven. However, this may not apply to everyone. There will be worlds corresponding to hell in which the soul suffers the consequences of its attachment to one or several of the various sins, and others more like limbo where the great mass of souls probably find themselves. Limbo is a kind of semi-material world that interpenetrates the physical and is like the astral plane or desire world of occultism. The astral plane itself has many levels, some of which may be dark and dull, others reflecting higher forms of consciousness but these are all the creation of the created which is to say the outgrowth of human desire, imagination and thought. In these worlds the soul when it has found its appropriate place will be happy with all earthly woes and problems removed. It can live as it likes but it will not know God or the higher spiritual realities though there may be an imitation of these to which the conventionally religious may gravitate, and here they stay until they experience the inner urge to move on and progress to higher levels. Some may even think they are in heaven for this is a state of natural happiness in which the earthly kinds of desires can be satisfied. But there is no real spiritual fulfilment and no consciousness of the presence of God. The conventionally good person may gravitate here as well as those who are still attached to the phenomenal side of life.
At higher levels we may find regions corresponding to the prelapsarian Paradise, and also to the heavens of the various pagan religions. The spiritual law is like attracts like so what you are is where you go. To get to higher/subtler realms you must work to eradicate that in you that acts as ballast, essentially sin and ignorance, and encourage the finer feelings of spiritual aspiration and attraction to what is truly noble and good. Part of this is developing imagination but not in the worldly sense in which we understand that word now. True imagination is receptivity to higher things. It is linked to spiritual sensitivity.
Paradise is spiritual but it is not divine, the difference being that it is still part of Creation. Above Paradise there is the true Heaven which is where the fully purified soul stands in the presence of the Creator who is now known as the very essence of your own soul. He is still God the Creator but he is is also fully immanent. There is no separation between you and him. However, Heaven is not a single place where every soul is the same as every other soul. If anything, souls in heaven are more individual not less so. They are now fully themselves, a unique aspect of God, now completed, but at the same time while every soul in heaven may be filled with God, there is always more God to be known and so every soul stands at a point in which God may be at the centre of their being but they can always move closer to the centre of God's being. The law of life is growth and this continues in Heaven as it must or else Heaven would have a lack in it. It is not pure being as opposed to becoming but being and becoming together, always working to create something more.
Here I must introduce the vexed question of reincarnation. As a believer in that mechanism for the education of souls I would say that most souls, after a spell in one or several of the inner planes, experience an urge to return to Earth to further their development in a sphere in which that is possible, the material plane of full separation. Those who ascend to Heaven after life in this world are probably fewer in number than we have been told. At either end of the human scale there are those who are damned and those who are fully saved but many souls, in my opinion, need to return and continue with their course of spiritual development in this world.
I will add a proviso. This might have been the pattern of the past. But we are at the end of an age. Now, souls may be facing an end of term examination in which their future paths are determined. It may be that the course is ending and souls must make definitive choices. That is my personal feeling. It also explains the vastly expanded world population. We are being called either to go up higher or, if we refuse to embrace that destiny, to fall back. The tests are coming thick and fast and it may well be that "wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: (but) strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it." Words to take seriously now more than ever when everything in the world is conspiring to lead us to the wide gate and broad way.