Atheists in church

[Spongebob]

...It's strange how atheists demand everything be rational and "true", but they also believe that death is the permanent end. Why do they waste time/energy defending an abstract concept like rationality, if everything is just going to disappear the moment they die anyway?

Because it is important while we are alive.

It's theists who claim that everything we do while alive on earth is only important in the context of an afterlife.  That should be the target of your confusion, not rational atheists.