They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 It was characteristic of ancient thought to see the world in terms of two conflicting principles. We see this very vividly in Zoroastrianism, the religion of the Persians. That was a religion with which the Jews had been brought into contact and which had left a mark upon their thinking. Zoroastrianism saw the world as the battle-ground between the opposing forces of the light and the dark. The god of the light was Ahura-Mazda, the god of the dark was Ahura-Mainyu; and the great decision in life was which side to serve. Every man had to decide to ally himself either with the light or with the dark; that was a conception which the Jews knew well. 