Q: What is forbidden in the ninth commandment?
A: The ninth commandment forbids whatsoever is prejudicial to truth, or injurious to our own or our neighbor's good name.
Proverbs 6:16-19 There are six things the LORD hates, seven that are an abomination to him: haughty eyes, a lying tongue, and hands that shed innocent blood, a heart that devises wicked plans, feet that make haste to run to evil, a false witness who breathes out lies, and one who sows discord among brothers.
From Starr Meade:
God only speaks what is true. God created us to be righteous and holy. He created us to speak and to love the truth as He does. Adam sinned by listening to Satan's lie rather than obeying God's truth. As a result, all of us are born as sinners who love falsehood more than truth. When God saves us and causes us to be born again, He changes us so that we want to live for God and please Him. He gives us a new love for the truth. Christians show that God has changed their hearts when they avoid falsehood and speak only what is true.
I think what is important here about this commandment is that knowing, speaking, and loving the truth is not simply about being "right". It's not simply about knowing and proclaiming facts. It's about loving what God loves. It's about reflecting God's character in our lives. When we indulge in lies, when we slander our neighbors, when we discredit our own good name, what we're really doing is declaring that we "love falsehood more than truth". And to love falsehood more than truth is to love a lie more than we love God. If we are truly desiring to love the Lord our God with all our heart, soul, strength and mind, and if we are truly desiring to love our neighbor as ourselves, then we will love and declare the truth, again not so that we can simply be "right", but rather, because we want to love God, and love the things that God himself loves.