Proverbs 14:5 A faithful witness will not lie: but a false witness will utter lies.
The meaning of this verse is clear. Have you ever been around someone who would lie when the truth would make much better sense. Lying seems to be a habit with some. Some people can hardly open their mouths without lying. I guess that is where the term pathological liar comes from. Sometimes lying can become almost like an illness in the since that they get so use to lying that it is as if they can't help themselves.
A christian should be truthful by the power of the Holy Spirit that lives within. We should be a faithful witness. If we say something, people should be able to trust us because they know that we will always tell the truth. We should have such a reputation for telling the truth that others will believe us and trust us. We should also do what we say we will do. If we are somehow hindered beyond our control from carrying through with our word, then we should apologize and/or make restitution. Even if we give wrong information because we were misinformed, we should always make things right. The bottom line is this: a liar cannot be trusted. We should always be truthful, a faithful witness.