The Eighth Commandment You shall not bear false witness against your neighbor.
Why should we not bear false witness? God is the source of all truth. His Word is truth. His Law is truth. His “faithfulness endures to all generations.” As God’s people, we are called to live in the truth.
Offenses Against Truth False witness and perjury Rash judgment: assuming as true the moral fault of a neighbor Detraction: disclosing another’s faults and failings
Offenses Against Truth Calumny: harming the reputation of others by remarks contrary to the truth Lying: violates the nature of truthfulness and does real violence to another.
Slander The evil tongue... (or slander) is considered among the Jewish people as one of the worst sins imaginable... one almost tantamount to murder in that the good name, livelihood, reputation, etc. can all be destroyed by a single word, look, expression.
How to Avoid Sins Against Truth To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor's thoughts, words, and deeds in a favorable way:
Don’t Be Afraid to Speak Up! The Christian is not to "be ashamed of testifying to our Lord" (2 Tim 1:8) in deed and word. Martyrdom is the supreme witness given to the truth of the faith.
One man spread a rumor about another. He later felt regret, and went to the rabbi to ask how to make amends. "Go to the store and buy a bag of seeds," said the rabbi, "then go to a big open field and scatter the seeds into the wind. Do so and report back to me in a week." The man did as he was told, and came back the next week to find out what to do next. "Now," said the rabbi, "go back to the field and pick up all the seeds." "But," protested the man, "those seeds have scattered far and wide! I'll never find them all. Many have even already taken root!" "Exactly," explained the rabbi. "Now you understand. When we speak badly about another person, the effect is far and wide. And it is damage that can never be fully undone."