Without question, ethical monotheism. The idea of one true god. The idea that our life and ethical conduct on earth determines how we will go in the next world. This has been responsible for most of the wars and bigotry in history.
But religion has also been responsible for investing countless lives with meaning and inner richness.
I lead a perfectly healthy, satisfactory life without being religious. And I think more people should try it.
(from What's the big idea?, an interview with Peter Watson)