The words that we speak as Christians tell us what is in our hearts. We may read Galatians 5 and read over the works of the flesh very quickly because we say to ourselves I don't do any of those things, but do we meditate on any of those works. Jesus let's us know that we don't have to physically commit the act to be guilty of the sin. If we have that act in our hearts then we have committed that sin. Jesus said in Matthew 12 we would all give an account for every idle word we speak. A person will give away what is in their heart by the words they speak. I have heard people say we cannot really tell if someone is a Christian or not because we don't truly know what is in their heart, but that is not what Jesus says. Jesus taught that we can know them by their fruit. We can know them by the fruit of their words and what they are saying.