I would have made a large reply about the religious aspects your brought up, but Kashis already pointed out what I wanted to say. That being that most religions don't support the claims of mediums and the likes that bring said religions into the mix. Furthermore, what if the afterlife is actually another dimension to which souls transcend our own reality and accumulate into new beings that we call ghosts? These ghosts being able to slip through dimensions on rare occasions and interacting with the world around us. Maybe they themselves are not the humans they look like, rather they take said form due to interactions with human memories that allow them to take form to begin with. Maybe they themselves are not human. Hell, maybe we make ghosts and the actions of ghosts with our own minds with some form of mental power that we have yet to discover. Really, the possibilities are endless and most of them do not need god(s) and the afterlife to be real.