Be a good example

If we live out our faith, others will see how true, beautiful, and desirous it is. By the same token, if we allow sin to damage our testimony, we tarnish the Gospel in the eyes of others.
We can't just "believe" our faith and principles or study them in our minds. They must become our lifestyle. When we "wear" our faith in everyday life, others will see our good works and glorify our Father who is in heaven.
People do not judge the Christian's testimony from the doctrine, but from his actions and life.