Well, wikipedia has left no stones unturned in explaining what afterlife is and how does it vary from religion to religion.
But why can’t there be any concrete theory on the definition of afterlife? Maybe because unlike any other matters of life in this world, afterlife is a matter not only “of” death but also “after” death.
Whether it be a spiritual or corporeal element of this world, if it could be experienced, it could be verified and justified, provided there are footprints left behind for such matters. Because footprints happen to be the facts. And it’s the facts that prove theories and thus bridging the gap between a notion and a notation.
But death doesn’t leave it’s footprints. It just sweeps away the dead. And oh! The dead cannot speak of its experience. So we are left with no first hand experience of what afterlife is in reality.
Humans have a tendency to go to greater extent to quench the thirst. Therefore, they tried to get hold of the “self near death experience” rather than “self death experience” making afterlife a “factual myth” .