In the Sermon on the Mount, Jesus taught His disciples basic gospel principles. As true followers of Jesus Christ we are commanded to set a good example for other people.
Ye are the light of the world. A city that is set on an hill cannot be hid.
Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house.
Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Source: Matthew 5:14-16; The Holy Bible: The New Testament (KJV).