Matthew 5:14 -
Ye are the light of the world. A city that is set on an hill cannot be hid.
1 Corinthians 13:1 -
Though I speak with the tongues of men and of angels, and have not charity, I am become as sounding brass, or a tinkling cymbal.
If a person is truly Christian, then it should be reflected in their actions and words. After all, the person is supposed to be new again in Christ, correct? Hence the term "born again"?
As we've had noted by others here, every person is an example to others of just what their faith represents and what they believe. If a person is a bad example, then they bring that much disrepute to their faith. If enough people are bad examples, then one has every reason to question the environment that could have produced them.