The power of light

And the light shines in the darkness, and the darkness did not comprehend it.

John 1:5

Always light has power over the darkness. There is no place on earth darkness can make the light ineffective. It is the otherway around.  Even a small light elimates the darkness in that place.
Similarly there is a spiritual light within us which eliminates the spiritual darkness that is around us.  Once we accept LORD Jesus as savior, His light comes to dwell in us.  What does that mean? His light means His nature, His love, His wisdom, His patience, His goodness, His kindness, His self-control, His Joy, His peace comes to dwell with in us. Wherever we go we carry this nature of God, His light within us and are expected to demonstrate it to those who don't have this light in them. We are expected to switch this light on within us. When we do then those who are around us with hate and evil and who suffer lack of peace, joy, goodness, who are sick and suffering will see the light and the evil nature which is affecting them is removed. They too receive the light.  Instead of switching the God given light in us, if we counter evil for evil, hatred for hatred it only makes the darkness to increase.
Apostle John witnessed and experienced this great light - our LORD Jesus Christ Himself. Hence he shares it as a testimony to us.  
Light always overcomes darkness. That is the power of light.