And neither do you.
Yes, yes, maybe you say that you "know", based on your holy book, or you had a "seeing the light" near-death experience, or heard from a dead loved one in a seance, or in your dreams. What you're really saying is that you have settled upon a version of the afterlife that fits into your worldview, gives you comfort, and allows you to make sense of an extremely unfair world. You perhaps have chosen to view certain experiences as evidence to back up what you believe will be your fate after you draw your final breath.
But no one knows.
What people thought happened to the dead changed and evolved through the centuries. The Jewish Bible is pretty quiet about any sort of afterlife, other than a couple of places. Two people, Enoch and Elijah are described as having been received bodily into heaven, while the prophet Samuel, or his ghost, is temporarily brought back to the land of the living to give some advice to the soon-to-be deposed-and-killed King Saul. Jewish thought around the time the New Testament was written tended toward a belief in a bodily resurrection at "the end of time" when God would overthrow the existing order and institute a "Kingdom of Heaven". Jesus seems to be an adherent of this view - his moral teachings very clearly intended to get people "right" so they would be able to enter the soon-to-be-established Kingdom. (Which he thought would come pretty darn soon)
After Jesus' alleged resurrection and ascension, and the failure of the end of the world to happen, the Apostle Paul put forth his theories. In some places in the Epistles he seems to go along with apocalyptic theology, describing a bodily resurrection at some future time. At other places he is apparently teaching that a Christian goes right to Heaven upon death. He never mentions Hell, but does write about wrath and judgement in many places. This lack of specificity left it to subsequent generations of Christian theologians to devise descriptions of Heaven and Hell, although the way they imagined Hell was a lot more graphic than that of Heaven.
Despite a paucity of official descriptions of what awaits us in Heaven, most people have at least a sketch of an opinion about what it entails. Usually a reunion of one's loved ones, living joyfully for all eternity. In my own experience, people envision their deceased family "looking down on them" and providing some kind of help, comfort or intercession while "up there". Like most religious beliefs, beliefs about the afterlife owe less to sanctioned dogma than to personal imaginings.
Beliefs outside of Christianity aren't any more concrete than the dominant Christian beliefs: feasting in the Halls of Odin in Valhalla, the Summerlands, reincarnation, Nirvana, getting your heart weighed against a feather, they're all based on some ideas that someone who wasn't dead thought about what happened to the dead. It's interesting to me to note that the Greco-Roman conception of the afterlife before Christianity was pretty dreary. You just shuffled around as a shadow (literally) of your former self in some dank underworld.
The truth is that if there is an afterlife, we don't know anything about it because it's not something we can investigate.
Belief in an afterlife is usually pretty harmless. If thinking that your loved one who died is "in a better place", or happily playing in an amateur harp combo, free of pain, etc, gives you comfort, then I have no problem with it. Historically, people have been conned into accepting pain and suffering during their lifetime because it would all be better "later". Personally I take the "I don't know and it doesn't matter" position. I'm going to live my life the best I can while I have a life. If it happens that there is an afterlife that I can consciously enjoy, cool, if not, I'll never know, will I?
