By the eighth commandment we are commanded to speak the truth in all things, but especially in what concerns the good name and honor of others.

Wherefore, put away lying and speak truth each one with his neighbor, because we are members of one another.  (Ephesians 4:25)

Saint John Nepomucene

The eighth commandment forbids lies, rash judgment, detraction, calumny, and the telling of secrets we are bound to keep.

Lying lips are an abomination to the Lord.  (Proverbs 12:22)

A person commits the sin of rash judgment when, without sufficient reason, he believes something hateful to another's character.

Before thou inquire, blame no man.  (Ecclesiasticus 11:7)

A person commits the sin of detraction when, without a good reason, he makes known the hidden faults of another.

A good name is better than great riches, and good favor is above silver and gold.  (Proverbs 22:1)

A person commits the sin of calumny or slander when by lying he injures the good  name of another.

Devise not a lie against thy brother, neither do the like against thy friend.  (Ecclesiasticus 7:13)

We are obliged to keep a secret when we have promised to do so, when our office requires it, or when the good of another demands it.

A person who has sinned by detraction or calumny, or who has told a secret he is bound to keep, must repair the harm he has done to his neighbor, as far as he is able.