Darkness = everything that isn't God, because God is light. Jesus conquered darkness, he was the Light of the World. Though he physically left the world, he told his disciples they are the light of the world.
3 possible reactions to this:
1. Incredulity: natural response of the meek. Jesus say it doesn't matter what age you are or what profession you have, he calls all believers to be lights in a dark world.
2. Resistance: object to the responsibility of being an example. Jesus says this isn't an option - if you're a Christian, you are a light.
3. Fearful: light exposes the dark, and those living in the dark may not want to be confronted with that. You will be persecuted for the sake of righteousness. Sometimes being exposed is the first step to finding healing. We may also be afraid of failure; it is a legitimate concern, but if we are truly doing God's will, he will be with us though the world may reject us.