Exodus 20:16 -- Do Not Lie

Exodus 20:16 Do not give false testimony against your neighbor.

In other words, do not lie.  Do not say something that is not true.  When you lie, you cause yourself to become untrustworthy.  Some people seem to think that lying isn't that bad.  Other sins, like murder or adultery are worse that telling a lie.

This is what the Bible has to say:

James 2:10 For whoever keeps the entire law, yet fails in one point, is guilty of breaking it all. 11 For He who said, Do not commit adultery, also said, Do not murder.

The one who said those things also said to not lie, so we must obey or we are just as guilty as the murderer or the adulterer.