That there are no gods - invisible friends aren't real, there's no afterlife, Jesus doesn't love you, the universe wasn't created for humans, ethics and morality weren't given from the sky etc.
(I don't know if it would actually improve the world. Maybe things would get better, but worse in the long run? Or vice versa? When is it measured from? Maybe most of the most terrible ideas, systems and inventions in history have been created to improve the world.)