Question: "Why is "You shall not give false testimony" in the Ten Commandments?"
Answer: Part of the Ten Commandments, also known as the Decalogue, this 9th commandment forbids the Israelites from bearing false witness or giving false testimony against one another (Exodus 20:16; Deuteronomy 5:20). To bear false witness against others is to lie about them, especially for personal gain. The Hebrew word translated “neighbor” in this commandment can mean an associate, a brother, companion, fellow, friend, husband, lover, or neighbor. In other words, the Israelites were commanded to be truthful in all things, but especially when speaking about another person. The people were not to lie publicly, as in a court of law by laying at another’s feet any false charge that could injure him, nor were they to lie privately by whispering, talebearing, backbiting, slandering, or destroying his character by innuendos, sly insinuations, and evil suggestions.
The reasons for God’s prohibiting lying and testifying falsely against one’s neighbor are three-fold. First, God’s people are to reflect God’s character. Jehovah is a truthful God who does not and cannot lie. Numbers 23:19 tells us, “God is not a man, that he should lie, nor a son of man, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?” The people who were called by God’s name and who represented Him in the heathen world were expected to accurately reflect His character. Lying to or about one another brought reproach upon His holy name, and this He would not tolerate. Second, bearing false witness against another was destructive to the individual who was the victim of the lie, and he suffered by it in his character, credibility and reputation, as well as in his trade and business. Leviticus 19:18 makes it clear that the Israelites were to love their neighbors as themselves, a command reiterated by both Jesus and Paul (Matthew 22:39; Romans 13:9). Loving our neighbors precludes lying about them.
Third, false witness was seen as so destructive to society that courts of law, both in the days of the Israelites and today, could function only if the witnesses who were called to testify could be trusted to tell the truth. Without a trustworthy judicial system, based on eyewitness testimony from reliable, truthful witnesses, societies are at risk of the breakdown of law and order. When this happens, chaos ensues and the innocent suffer.
As noted before, the New Testament is equally condemning of false witness. Colossians 3:9-10 explains the reason for the continued prohibition against lying. Christians are new creations in Christ (2 Corinthians 5:17) and as such, we reflect His nature. We have been released from our “old self” with its evil practices such as lying and bearing false witness. As the Israelites were to reflect the character of Jehovah, Christians are to reflect to the world the character of Christ that identifies us as His own.