It is believed by the theological scholars of Therafim that there was never a true beginning, and that there will never be a true ending. Time is a convenient illusion for both mortals and gods. The Light and Chaos split from each other at the point that is generally known as the Beginning, but this is simply a reference point in the chronicles of time, used to identify the point past which neither mortals nor gods can extend their understanding. The Light or Chaos might know more, but those deities do not share secrets save with those who join them at the final heights of Heaven or the lowest depths of reality itself. Since those who do manage to join these deities in this manner are never heard from again, however, the secret is generally held to be beyond common comprehension. There are rumors, however, of existence before Chaos, and of the powers that brought reality into being with the split between the Light and Chaos acting as the initial catalyst.
Originally, after the initial rift between the power of ultimate creation and ultimate destruction, there was left a space in the very middle of all that is. That space was called Paradise. Paradise was the place where the gods and all that lives now first separated themselves from the netherstuff of raw creation, and took on individuality. This place was intended as the spawning and developing ground of all that would eventually become real, and thus nothing within Paradise had substance as we understand it. Paradise was outside of both time and space, a place of pure innocence, because time and space are required to give definition to all that exist. Paradise was the cosmic womb, and the souls of all that have been, are, and will be spring from this primal origin point. The purpose of all of these souls, all this potential life, was to discover for themselves what they wanted to make of reality, and then to achieve enlightenment by pursuing what they felt would bring them the greatest happiness.
Eventually, however, Paradise was ruptured by conflict. There were many souls at this time that rebelled against the entire order of reality itself, and in their rebellion they became dark and terrible. These were the first Fallen Ones, as well as others of similar primal evil. In the end, the reason for their rebellions do not matter so much, because no matter what their reasons might have been, they were cast out of Paradise - out of all of standard reality, in fact - by their own actions, no longer able to remain within a realm of creation that they had rejected. These were the first seeds of discord which eventually influenced Bahamut and Tiamat, the greatest of the souls among the many that lived in Paradise. These two great ones were drawn to each other, like attracting like and opposites attracting opposites. It was only a matter of time before their acts of lovemaking and conflict caused them to fall out of Paradise, no longer able to dwell outside of the bounds of reality, and into the realm of time and space, which is reality as we understand it. Their first children are the other greater gods, and it is by the will of these greater gods alone that any others may be admitted into the extended family of the gods. From the gods sprang lesser creatures, and most importantly the creatures of the Prime Material Plane. The gods recognize mortals are a critical part of their own quests for personal enlightenment (though what this means varies based on the individual god), and this, as well as the power that comes from mortal faith, is why the gods are so interested in the struggles and daily routines of mortal creatures.
The Prime Material Plane, in all its infinite variety, is where all that lives is tested and proved and given an opportunity to make choices. These choices eventually determine the ultimate destiny of each living soul, and it is the principle duty of the gods to provide guidance to mortals, to offer them choices for them to make that will eventually draw them upwards or pull them downwards, wherever a given deity feels true enlightenment eventually lies. This makes the mortal planes a vitally important part of reality, where the conflicts that determine the very essence of existence itself play out in all their many hues and shades of drama and excitement. Not infrequently, mortal souls are reincarnated, or sent back into a new mortal form that will help them gain a better understanding of their true desires, until, eventually, their ultimate characters and natures have been decided, and they are ready to move on to the Outer Planes to seek additional opportunities for enlightenment. Unless this is done with a spell, however, the reincarnated individual does not retain memories (and hence levels and related abilities) from prior reincarnations, since divinely-appointed reincarnation is supposed to give a mortal a chance to start fresh for a clearer perspective; the memories from each prior life are restored when a mortal soul is finally ready to move to the Outer Planes. It should also be noted that most mortals are not reincarnated, but instead come from the seemingly limitless boundaries of Paradise.
In the cosmology of Therafim, the Transitive Planes are the places where Paradise initially spills out the souls of all that lives in some form. These spaces are intended as the starting point for new souls, and the travel point for souls on their way to other destinations. As they are intended as intermediaries between other places, rather than end destinations in themselves, of course, they are often used by mortals and immortals alike as pathways that makes more mundane transportation possible. Some of them are also places where souls can get stuck between worlds, usually because of ties that keep them partially tethered to the mortal realm. Dreams take place in the Transitive Planes, and because of its ephemeral nature, the world of the fey is also found here, partway between the Prime Material and Outer Planes.
The Inner Planes are where the stuff of reality is drawn and where it achieves its ultimate expression, providing the building blocks for the rest of the planes. The creatures of the Inner Planes, because of their nature, have a strong interest in extending themselves beyond their plane of origin, and it is for this reason that elemental beings and similar creatures tend to find their way so readily onto the Prime Material Plane, where they feel most useful and alive. These elemental beings have always existed as they are, and if they are slain will simply merge again with their home plane and eventually be reborn once more until they have achieved sufficient experience The Plane of Shadow, incidentally, is one of the Inner Planes in Therafim's cosmology.
Meanwhile, the Outer Planes are intended as the places where the souls of mortals go after they have had sufficient opportunity to make decisions for themselves, and are now ready to properly follow the path of enlightenment that they have chosen. Whether an individual's version of enlightenment leads them to heaven or hell, however, is a very personal sort of decision, but it always results in mortal souls going to the place where they feel most comfortable, most at-home. Extraplanar beings of this sort often retain memories of their mortal lives, even after they have transformed into a 'native' of a given Outer Plane, though these memories tend to be hazy and indistinct, depending on their strength and importance to the present nature of the extraplanar in question. It is on the Outer Planes that the gods are able to work directly with their followers to gradually make their way towards the final enlightenment that all creatures seek, when they are not entangled in the many conflicts that plague them. Unless conflict is a part of their version of enlightenment, of course.
On Therafim, there is far less ambiguity about what the gods love and what they hate, as they are an active and very real presence, and almost all communities will have some worker of divine magic. While the gods do not provide magic to the world (that is an inherent part of Therafim itself, and part of what makes it such a highly-desired prize by so many forces), they are able to channel it to their worshipers in a controlled fashion that is far easier to gain and vastly safer to use than the arcane energies wielded by mages (and the reason why clerics on Therafim generally start adventuring at ages more appropriate to less scholarly classes). The gods are much more restricted in the amount of direct intervention that they can take with mortals, requiring them to either work through Umnos' legitimately established channels, or else to break divine laws and reap appropriate consequences therefrom, and this is why they are so eager to work through mortal proxies, providing such ample demonstrations of their power. Thanks to this high prevalence of divine magic on Therafim, the world is quite a different place than Earth in similar medieval times, with far less disease, higher rates of birth, and far healthier people in general, which has allowed all the races of Therafim to be vastly more productive.
This is just as well, of course, because Therafim is also an incredibly dangerous place, the raw forces of magic and creation still rampant in the world leading to similarly enhanced health and rates of birth among monstrous creatures, which are a constant menace. These raw creative energies are also at work inside the world of Therafim itself, causing mineral wealth to gradually regrow in their veins, provided that they are carefully tended (a regular task of druids in the Underdark of Therafim, especially among the dwarves), and so there is more than ample treasure on Therafim, for those who are willing to take the risks necessary to acquire it. These raw, creative energies also mean that the elements themselves are frequently more extreme, requiring mortals to be especially careful at the times when the seasons are changing, and hence at their most volatile. Finally, because of the meddling of the gods in the lives of mortals, mortals are very frequently drawn into conflict with each other as a part of the desires of the gods to achieve enlightenment and to bring all mortals with them on their chosen path to that ultimate goal.
Thanks to this combination of great danger and vast treasure, the strange class of people generally called adventurers are a necessity on Therafim. The gods of Therafim approve of these unknown elements, though they are also wary of them, because of their capacity to work so much change, and so adventurers are considered a prime prize for the gods, and they are often given special consideration in the granting of powers and when exceptions are needed to explain their behavior.
The deities of Therafim are what they are, and they are the final forces to whom all those on Therafim must answer when deciding one's fate after mortal life has finished. Mortals are their agents on the Prime Material Plane, and their energies draw directly from the power of their worshipers' faith, as well as the raw creative potential that is a part of Therafim itself. While some may be less faithful worshipers of the gods than others, there are none except the willfully self-delusional who would deny their existence and the power and benefit that they bring to Therafim and its peoples. Because of them, the world is a better place. Without them, Therafim would be far less wonderful.