If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth. But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin. (I John 1:6-7 NKJV)
This couldn't be any more blunt then what it is. The problem is that many Christians have compartmentalized their lives. Church is where they are spiritual. In the privacy of their home when no one is looking they look at things on the internet they shouldn't and maybe do other things they wouldn't think of doing in front of their Christian friends. At work and out in the world they are worldly. They talk like the world, look like the world and act worldly.
I'm not saying Christians should be legalistic, just different than the unsaved, ungodly person. Their lives should be consistent across the board...walking in the light of God's Word all the time.