We already have a topic like this
here, but I guess I'll put a quote anyway.
"You said to yourself last spring, 'In my view it is not necessary to destroy anything, all that need be destroyed in mankind is the idea of God... Once mankind, each and individually, has repudiated God (and I believe that that period, in a fashion parallel to the geological periods, will alive), then of its own accord..., the whole of the former morality will collapse, and all will begin anew... Every man will discover that he is wholly mortal, without the possibility of resurrection, and will accept death proudly and calmly, like a god. Out of pride he will grasp that there is no point in him complaining that life is a moment, and he will come to love his brother without any need of recompense. The love will only be sufficient for the moment of life, but the very consciousness of life's momentariness will intensify its fire just as much as it formerly ran to fat in hopes of an infinite love beyond the grave' ...well and so on, etcetera, etcetera. Most charming!" the devil quoted.
Ivan sat with his hands pressed to his ears and looked at the floor, but began to tremble in all his body. The voice continued:
"Now the question, my youthful thinker thought, was as follows: is it possible that such a period may ever arrive? If it does, then all is decided, and mankind will finally save itself."--
The Brothers Karamazov by Fyodor Dostoevsky, page 829, from the chapter "The Devil. Ivan Fyodrovich's Nightmare."
I put that quote because it is from one of the most amazing scenes in a book that I've ever read. For 800 pages, a character named Ivan made his life revolve around his intellectualism and his belief that God was the destroyer of mankind. In this scene, he hallucinates the devil sits and talks to him. The devil mocks Ivan about his atheism, and unravels his intellectual arguements. His intellect being the last straw of his sanity, Ivan is finally driven insane.