Argument Against the Possibility of an Afterlife

Reductio ad Absurd um:
Telling a religious person that they are wrong is pointless. Instead follow them in there hopes and dreams and illustrate the conclusions. What are the ramifications of eternal life?
Eternal paradise is an individual concept, obviously. But it is always based on the epitome, the peak of happiness in his or her life. If that moment were stretched on into eternity, it would become mundane, meaningless. If it were a single moment stretched to eternity, it would still seem like only a moment.
No matter how varied and imaginative your activities, they are all based on the same governing rules that define the universe, if for no other reason than your mind isn’t capable of experiencing anything else. It is a product of the universe, and as such, is wholly endemic to it, and dependent on it for all experience, function, and even imagination. Say there is suddenly no such thing as gravity, and you can float and fly as you please. This is still based in reality though, because it is only in comparison with a concept, a rule of the universe.
Unfortunately for the religious concept of eternal life, the human mind is geared to operate on the concept of change, of cause/effect, the passing of experiences defines time, and the pace of time. The untrained human mind is trapped between the conscious realization of the unknowable future, and the animalistic tendency toward self-preservation. It naturally tends to seek constancy and eternity as a comfort from the unknown.
Once constancy is found, though, the mind tires of it. There is no change introduced, and the mind stagnates and submits to entropy. The whole world appears from the mind’s perspective to freeze up and lose motion, focus, and meaning. When faced with true eternity the untrained mind begins to understand, and recoils in fear.
I believe that no matter the state of eternity, whether it be heaven or hell, after a billion trillion years of unchanging constancy, the mind will be reduced to a wisp of neutral gray emotionlessness, beyond even the pain of watching all your hopes, dreams, fears, loves, ideas, knowledge; your very existence become meaningless and inconsequential in the infinite halls of heaven or hell.

Eternal life is an individual concept that is incompatible with individuality.

I could compare life to an endless field of green grass…

You are a human. For a while, though, lets pretend you are a beetle.

When you are born, you are a beetle in the soil below the grass, staggered by the richness, depth and complexity of the vast green forest. You are comforted by the dirt around you, and frightened by the height of the tall stalks. You are glad of the sunlight that shines through the dense grass above, and terrified of the dark corners and hidden depths concealed within the menacing blades.
As you grow older you are the beetle that climbs up the stalks of grass. You see the other creatures that you share the field with, and view their relationship with the green stalks and the rich earth. You feel the great winds that blow above the sea of grass, and seem to make the whole world sway and rustle. You see more of the field now, and have a greater understanding of how it works. You are staggered once again by its beauty and complexity, but terrified of it now that you see more of its unkindness and brutality.
As you grow even older you are a beetle on the wing, flying above the endless field. The great winds now just ordinary breezes that buffet you from time to time. Your eyes have been uncovered, and you see the endless green expanse for what it truly is. You watch as the gusts cause the green ocean to ripple and flow beneath you like liquid jade. You are staggered to your tiny beetle knees at the majesty and complexity that is your world. You understand now that the happiness, sadness, bravery, and cruelty that you had seen down in the blades was not just or unjust. It wasn’t part of some greater plan, as if there were a point to be made by the field existing. You realize that the field existing is the point. It is all simply the rolling of the wheel, the blowing of the great wind. Unfortunately some beetles get blown to the ground, or caught in the wheel’s path.
As you grow ever older, you fly ever higher. It becomes harder for you to make out the individual blades of grass, or the other inhabitants of the field. It becomes harder and harder to know how high you’ve flown. The dimples and ridges that broke up and defined the grass field seem to level out and disappear. The endless grass field just looks like a vast green plane; a flat expanse whose smooth uniformity makes it hard to determine whether you are miles away or merely inches.
As time goes by you grow bored. You remember the grass, and what it was like to roam through it and chew on it. You remember what it was like to skim across the top of the grass stalks, feeling the tips brush lightly on your antennae. You want to go back down to the field and experience it all again.
But you can’t. You are not a beetle, you’re a human. You are a conscious being. You cannot go back down to the field. You cannot erase your memories and go back to childhood, or adolescence, or early adulthood. You cannot “rewind.”

When a person wishes for eternal life, they truly wish for eternally new life. Because we have memories, we can only experience new life once. The trouble is, without memories we could never appreciate and enjoy, fear or hate life, we could only experience it. And if we could only experience it, we would be a beetle, and would never wish to live forever.