I understand the need to be a good witness of the faith, to set an example for others, to show that it is through the Holy Spirit that we are changed. However, it is only yesterday that I found the exact verses and understood the fundamental reason to be a good witness:

Matthew 5:14-16
You are the light of the world. A city that is set on a hill cannot be hidden. Nor do they light a lamp and put it under a basket, but on a lampstand, and it gives light to all who are in the house. Let your light so shine before men, that they may see your good works and glorify your Father in heaven.