Light is useful for many things: safety, helping people see what's real and what's false, helping things grow, guiding, warming, etc.
In a contest between darkness and light, light always wins! As soon as the smallest light is turned on, the darkness is gone. The difference is how much light is present - some rooms/places are lighter than others and you get more of the benefits of light because of that.
John 8:12 - Jesus says whoever follows him will never live in darkness. By this he means surrounded by evil, untruth, or injustice - whether the person themselves is responsible for it (by committing acts of sin), or the victim of it.
John 3:19-21 - Darkness and sin are related: men loved darkness more than light because their deeds were evil. We're more likely to do wrong, and get away with it, when what we're doing can't be seen..
If a person has "the light of the world" in them, does that mean they will sin less? In a way - light exposes evil, makes you more aware of your sin. But it doesn't guarantee you'll act on that knowledge, and it might not happen right away. Old, bad habits might not seem wrong if you've grown accustomed to them; even after you know they're wrong, it can take a long time for you to stop.
Matthew 5:14-16 - You have the light, so shine the light, don't hide it.
So if light will chase away darkness, why is that so hard?
Sometimes we don't want others to know we have the light, sometimes we're not confident in the power of the light. Other Christians need to be together to maximize the impact.
Example: if trying to turn on 12 flashlights, only one at a time, in silence, it's not likely to be successful - unless - you coordinate efforts. In the same way, Christians need to know other Christians and combine their lights to fill the world with light.
