The truth will set you free.

In the oldest work in which I've found that phrase, Jesus is reported to have said, "If you hold to my teaching, you are really my disciples. Then you will know the truth, and the truth will set you free."*

Regardless of the intended meaning in that quote, to me the truth setting someone free means that when the truth is known, there is nothing left to hide. When I tell the truth then it's not necessary to try and remember what lies have been told to whom, or to worry that someone will catch me contradicting myself, or to feel guilty about deceiving someone. Honesty frees me from those unwelcome burdens.

*The Gospel According to John, Chapter 8, Verses 31-32