From Freud

"Sigmund Freud's argument, though often shocking to believers, is consistently logical: If religion is an illusion, it is the greatest of all illusions, in fact, a species of collective insanity, like the imaginary friend of a child who never grew up. The same is true, of course, about atheism if theism is true: It is the child's denial of the parent's existence." from Peter Kreeft in the introduction of Does God Exist?.