1 Corinthians 9:27

But I discipline my body and bring it into subjection, lest, when I have preached to others, I myself should become disqualified.  

Jesus came to this earth to save us and show us how we are to live.  We are representatives here are earth, and through our knowledge of the word, we become examples.  Our integrity then is an essential witness to the non-believer.  We should be consistent in our faith and not conform to the pressures of the world but follow the example of Jesus.