Because we humans are self-willed and prone to tell and embrace lies, we have to help each other by pointing out where we've gone off course.  This is called a rebuke, and few Christians do it well.

There is a fine art to rebuking someone.  Generally speaking, it is important to not shame the person, so a rebuke should almost never be in front of others or in such a way that the person is left to lick his wounds.  Let's look at how Jesus did it.*

One day, a Pharisee insinuates that Jesus is a phony.  How will Jesus respond?  Does he go after him with his rhetorical bazooka and call him a white-washed sepulcher?  How to confront this man?  Jesus doesn't want to shame him and in fact wants to help him grow.  Jesus deftly accomplishes this with a quick story and a question:

"Simon, I have something to tell you."

"Tell me teacher."
"Two men owed money to a certain money lender…which of them will love him more?"
After Simon answers, Jesus affirms him: "…You have judged correctly"

From this brief interaction, we can extract the following five principles:

1. A rebuke is required or a lie will persist.  The process of spiritual growth involves weeding out lies. We live unaware of them.
2. Get permission.  A rebuke is more effective when permitted.  Our walls of self-defense are only lowered voluntarily.
3. If possible, package it with a story.  People like to be told stories.  Stories shine the light of truth, but rather than a spotlight, are a softer, indirect light.
4. Bring it home with a question.  Questions are an invitation to participate in the discovery of truth. They give dignity.
5. Affirm the person.  Affirmation is like cauterizing a wound.  It prevents the infection of further lies like, "I'm bad."

*Luke 7:39