Romans 2:21

Well then, you who teach others, do you not teach yourself? You who preach against stealing, do you steal (in ways that are discrete, but just as sinful)?

Christians are supposed to be an example of the faith. If we go to Church on Sunday and do not follow the teachings after we walk out the door, no one will want to follow our example. If we do not adjust our hearts to believe in the Word, our actions will not line up. Sin is no friend to the Christian, and we should turn away from every opportunity to sin.