Imagine someone that you know asks you for a glass of water; so you hand them one. There is only one problem, they just witnessed you spit in it. I don't know about you, but spit isn't one of my favorite ingredients. How do you think that person would react? Some may take it as a joke, some may get angry and throw it in your face. No one would drink from that cup unless they were on the verge of death.
Similarly, When we are supposed to help someone learn about Jesus and be a light in their life and they constantly see us in sin we are destroying our witness. How can that person that you are supposed to lead to knowing Christ take you seriously if they see you practicing the opposite of what you preach? It has been said that our lives may be the only Bible that someone ever gets to read.
2 Corinthians 3:2-3
Are there things that we are doing in your life that could be a stumbling block for other believers/converts? Perhaps there is something that you are doing that you may not look at as 'sin' but it can potentially hinder your influence. What are we ready to rid ourselves of to be more effective in Christ? Is God saying the things that you are watching on television? Or perhaps the things that you are listening to. You will have to search within your on heart and pray to find out what it may be.
Just remember the message that we are called to bring to people's lives can be a fresh cup of water, we must not taint it.