In chapter 17 of Brave New World, John the Savage and Mustapha Mond discuss the emotional need for God.
John asks the World Controller, "...isn't it natural to feel there's a God?"
Mustapha Mond replies:
"You might as well ask if it's natural to do up one's trousers with zippers," said the Controller sarcastically. "You remind me of another of those old fellows called Bradley. He defined philosophy as the finding of bad reason for what one believes by instinct. As if one believed anything by instinct! One believes things because one has been conditioned to believe them. Finding bad reasons for what one believes for other bad reasons–that's philosophy. People believe in God because they've been conditioned to.
So, Mond's response is that the Utopians have no instincts, at least not for God. He says that he has conditioned the citizens of the Brave New World not to be unhappy; therefore, there is no need for God.
Mond finds the idea of God emotionally unsatisfying. He thinks that the desire to seek God is a kind of emotional response to suffering and pain. Since he has eliminated all suffering and pain, he has eliminated the instinct for God by extension. God is like a zipper. One doesn't have the need for it if he doesn't know it existed in the first place.
Mond believes humans to be empty vessels. Before and shortly after birth, his labs fill them up with all the feelings and thoughts they will ever need. Instinct is to be averted, like books, nature, and family. All instinct is replaced by genetically engineered "bliss."