Here's One Way to Explain Sin to an Unbeliever

In this short excerpt from a questions and answers session at our 2014 National Conference, R.C. Sproul suggests one way to explain sin to an unbeliever.

Question: Both of my brothers are unsaved. One believes there is no such thing as sin; therefore there is no need for a savior. How do I answer this question? How do I explain sin?