Felix Adler who founded Ethical Culture warned about the certainty of religious fanatics. Certainty can lead people to condemn heretics and burn witches in the fires of fanaticism. Crusades, inquisitions, and jihads have slaughtered men, women, and children. Some true believers are eager to kill each other to prove a point. That’s why Adler founded a movement based not on competing metaphysics, but on ethical cooperation. Atheists can be certain too, but usually they don't kill because of it.