But whoever keeps his word, God's love has most assuredly been perfected in him. This is how we know that we are in him:

 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. 