Q. 112. What is required in the ninth commandment?
That I bear false witness against no man, nor falsify any man’s words; that I be no backbiter, nor slanderer; that I do not judge, nor join in condemning any man rashly, or unheard; but that I avoid all sorts of lies and deceit, as the proper works of the devil, unless I would bring down upon me the heavy wrath of God; likewise, that in judgment and all other dealings I love the truth, speak it uprightly and confess it; also that I defend and promote, as much as I am able, the honour and good character of my neighbour.
 Proverbs 19:5, 9 and 21:28;  Psalm 15:3;  Romans 1:29–30;  Matthew 7:1, &c., Luke 6:37;  Leviticus 19:11;  Proverbs 12:22 and 13:5;  1 Corinthians 13:6; Ephesians 4:25;  1 Peter 4:8.
The Ninth Commandment, “Thou shalt not bear false witness against thy neighbour,” has to do with the maintenance of truth between man and man. While it may be quite shocking for a believer to discover that a hitherto credible professor of faith has been deliberately spreading falsehoods, experience teaches us that many unbelievers tell lies routinely and unashamedly. How many of us, who are believers, have been taken in by the smooth talking of unbelieving salesmen, to think that they were telling the truth only to discover too late that we have been cheated in one way or another? How many of us have felt hurt because we were too naïve to think that no one can lie straight face?
The sad truth is that men are by nature liars. Men, by nature, are held captive to sin and Satan, who is the father of lies (Jn 8:44). And since lying, unlike murder or adultery, often leaves no evidences, many an unbeliever, accustomed to the ways of the world, will lie without his darkened conscience discernibly indicting him.
But falsehood is not just contained amongst unbelievers. It can also be a constant temptation to believers because of the remnant of corruption. For this reason, it is necessary for believers to be well acquainted with the many ways in which we may fall into this sin, so that we may avoid them and also flee to Christ in repentance when we discover their appearance. We must do so bearing in mind that all lies and deceit are “the proper works of the devil,” which do incur the “heavy wrath of God,” though they may not be discovered by our fellow men in our lifetime.
Q/A 112 gives us a succinct summary of the many ways we can break the Ninth Commandment. Firstly, we break the commandment by bearing false witness against another person or falsifying his words. That is to say, we sin if we say something false about someone or about what he said to another person or persons, whether in informal or formal situations. Secondly, backbiting and slander are also a breaking of the Ninth Commandment. This is related to the first point, but slander and backbiting involve not just falsehood but malicious character assassination. Thirdly, it is also a breaking of the Ninth Commandment if we form judgments about another person based on unsubstantiated hearsay.
The Ninth Commandment, of course, is not about not promoting falsehood. It is also about promoting truth and justice. As God is truth, the Christian must love the truth and seek to maintain it at all cost. We must maintain not only the truth of the Gospel, but seek to defend and promote the good name and honour of our neighbour. Thus we must never be party in spreading unfounded rumours. We must rather seek to correct falsehoods about someone that we are aware of, once it comes to our ears.
Heidelberg Catechism >