Thou Shalt Not Bear False Witness

  You shall not bear false witness against your neighbor.

Briefly stated, this means that we should fear and love God that we may not deceitfully misrepresent, betray, slander, or hurt the reputation of our neighbor, but defend him, speak well of him, and put the best construction on everything.
Besides our body, our spouse, and our temporal possessions, we have yet another treasure, our good name and reputation. A respected name and good reputation are of great value, for most men cannot bear to live among others in sin, open shame, and contempt. Therefore, by this commandment, God protects our good name and reputation. We are not to undermine the good name of our neighbor, harm his reputation, or cast doubt on his character or integrity. The primary application of this commandment is, as the words imply, in the courts of justice. We are not to falsely accuse or bear witness against anyone.
To most people the possibility of being called upon to testify in court seems remote and of little concern. However, our sinful nature being what it is, whenever men see a chance to gain by lies and deception some will try to do so. To such men, lying under oath is just another way of convincing people that they are telling the truth. When such lies are told the poor often cannot afford to defend themselves, and the innocent, both rich and poor, suffer. This situation would not be so tragic if godly men presided in the halls of justice, however, that is rarely the case.

  Above all else, the Bible requires a judge to be a godly man. He must also be wise, modest, brave and bold. A witness must likewise be a fearless and godly person. For a person who is to speak the truth, judge all matters rightly, and carry them through with his decision will often offend good friends, relatives, neighbors, and the rich and powerful, who may greatly injure him. Therefore he must be quite blind, have his eyes and ears closed, neither see nor hear, but go straight forward in everything that comes before him, and decide accordingly.
Therefore, this commandment is given to protect our rights. It calls upon us to help our neighbor to secure and protect what is rightfully his. We should not allow justice to be twisted and perverted by technicalities. All who are involved, judge, jury, and witnesses have a responsibility to deal honestly and uprightly with every case. We are to allow right to remain right, and should never portray wrong as right. Woe unto those who call black white and white black! We are not to gloss over the truth, keep silent about certain facts, or pervert justice because of a person's status, race, or wealth. Thus, this commandment applies to all that takes place in court.
However, this commandment has a spiritual application that extends far beyond its use in court. In some way, everyone bears false witness against his neighbor. One common occurrence of such false witness is seen whenever godly preachers and believers are ill spoken of, slandered, and falsely called hypocrites, heretics, or apostates. It is also seen whenever falsehood is presented as truth, whenever men call their own ideas and private interpretations the Word of God, and whenever the Word of God condemned, contradicted, and denounced. Such hostility to the truth is to be expected, for the way of this world is to condemn and persecute the truth and the children of God.
This commandment also forbids all sins of the tongue that might injure our neighbor. False witness is, after all, a work of the tongue. God prohibits every use of the tongue against a fellow man, whether it is false preachers with their false doctrine and deceit in God's name, or false judges and witnesses with their unrighteous verdicts, or just plain everyday evil talk and lying. Included here is the vice of slander, cutting someone down behind his back. The devil spurs people on in such behavior, and it is an evil plague. People much prefer to hear bad things about their neighbor than good, but they would be outraged if they caught someone speaking in the same way about them. We want the world to speak of us in glowing terms, but would resent it if they spoke of our neighbor in such terms.
In order to avoid this vice, we should make it clear that no one who is not in a position of authority is allowed to publicly accuse and reprove his neighbor. If you think that you see your neighbor sin, do not assume that you have all the facts relating to the situation, it is easy to misinterpret someone's actions. If you are convinced that wrong has been done, either go to the authorities or discuss it with your neighbor privately. Even if your neighbor has done wrong, you have no business proclaiming it to all the world or stirring up trouble behind his back. You are not the judge and jury, therefore, until you are appointed judge, either keep what you know to yourself or go through the proper channels.
Back biters are not content with knowing of an offence, but assume an air of authority, proceed to publish the fact, and denounce the guilty party as if they were judge, jury, and executioner. They are delighted and tickled that they can drag their neighbor down and stir up trouble for him. They enjoy digging up dirt the way a pig enjoys rolling in the mud or rooting through it with his snout. Such people are attempting to usurp the role of God by pronouncing the sentence and prescribing a most severe punishment. Although they do not have the right or authority to carry out that verdict, they employ their poisonous tongue to shame and hurt their neighbor.
This is my question to those who think that they have the right to publish every fault of their neighbor. If you are so sure of yourself, why don't you contact the authorities? Why don't you go through the proper channels? Is it because you are not certain that you are right? Is it because you are afraid that you will be rebuked and embarrassed? Is it because you cannot prove your accusations? Well then, as long as you cannot prove it, and are unwilling to go through the proper channels, you ought to keep your mouth shut. If you cannot prove it, how do we know that you are not a liar? A man should be regarded as innocent until proven guilty. We do not have the right to deprive anyone of their honor and good name until they are properly convicted.
Therefore, this commandment forbids a witness that does not rest upon sufficient evidence. We should never jump to conclusions about our neighbor, or declare our own assumptions about his behavior as the truth. Do not accuse anyone until you honestly know what you are talking about. Even then, go through proper channels and let the authorities decide the matter. Until this has been done, either keep the matter to yourself or discuss it privately with the party in question. If you encounter a person who speaks ill of another, rebuke him to his face so that he will hold his tongue. He has no business slandering someone, or speaking before all the facts are known. Once a good name has been destroyed, it is not easily restored.
While this commandment forbids us to speak evil of our neighbor, it should never be interpreted in a way that would allow evil to go unpunished. Those in authority, rulers, preachers, and parents, are allowed to publicly expose and condemn evildoers when it is done in the line of duty. We are forbidden to kill, yet the executioner, who by virtue of his office does his neighbor only evil and harm, does not thereby sin against God's commandment. He is blameless because God has instituted the office of executioner as an instrument of His wrath. While the Bible condemns immodesty, a doctor must sometimes be allowed to examine the private parts of the patient whom he is to cure. Likewise, while we, as individuals, do not have the right to judge and condemn anyone, if they whose office it is to judge and condemn fail to do so, they sin. In that situation, necessity requires one to speak of the evil, to prefer charges, to investigate and testify. In the same manner, father and mother, brother and sister, and other good friends are under obligation to each other to reprove evil wherever it is needful and profitable.
In dealing with actual offences, the best procedure to follow is that given in Matthew 18:15, where Christ says, "If thy brother shall trespass against thee, go and tell him his fault between thee and him alone."  This is excellent instruction for governing the tongue and preventing its misuse. Therefore, let this be your rule, do not too readily spread evil concerning your neighbor, or slander him to others, but admonish him privately that he may amend his life. Likewise, if some one tells you what someone else has done, teach him to go to that person and discuss it privately or else hold his tongue.
This approach to dealing with people is only common sense. When a supervisor at a place of business sees an employee who is loafing and not doing what he should, he admonishes that employee personally. Could you imagine what would happen if that same supervisor allowed that employee to continue loafing while he went out and complained about him to people on the street? He would no doubt be told, "You fool, what does that concern us? Why do you not tell it to him?" They would be fully justified in rebuking such a supervisor in that way. That is the only way the wicked behavior could be stopped. In Matthew 18 Christ also said, "If he hear thee, you have gained thy brother." What a great and excellent work! It is no little matter to gain a brother. Let all monks and holy orders step forth with all their works melted together into one mass, and see if they can boast that they have gained a brother.
 Further, Christ teaches, "But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established." So we are always to deal with the offending party personally instead of talking about them behind their back. However, if you still cannot get anywhere, bring it publicly before the community. Depending upon circumstances, this could be either the civil or the congregational authorities. Once this is done you no longer stand alone, but have with you the witnesses needed to convict the guilty party. On the testimony of those witnesses, the judge can pronounce sentence and prescribe punishment. This is the right course to follow in dealing with, and reforming, a wicked person. But if we gossip about him and dig up dirt, no good will come of it. Worse yet, if you gossip and slander, then afterwards, when called upon to bear witness, admit that you did not know what you were talking about, you would deserve to be severely punished as a warning to others. If you were acting for your neighbor's reformation or from love of the truth, you would not sneak about secretly, but would instead deal with the problem in a godly way.
All this has been said regarding secret sins. However, when the sin is already public knowledge, so that the judge and everyone else knows of it, there is no need for you to deal with the offending party privately. He has brought himself into disgrace, and you may publicly testify concerning him. When a matter is public, there can be no slandering, or judging and testifying falsely. That is why we are now free to reprove the Pope. His false doctrine is public knowledge, it is publicly set forth in books and proclaimed throughout the world. Where the sin is public, the reproof also must be public, so everyone may learn to guard against it.
In summary, this commandment forbids us to harm our neighbor with our tongue. If what is spoken is not in accord with the instruction of God's Word, (for his reformation) it does not matter if he is friend or foe, or if what is said is true or false. We are not to undermine our neighbor's reputation or air all of his faults. Instead, we are to speak of our neighbor in the same way we would have him speak of us, for Christ has said, (Matthew 7:12), "Whatsoever ye would that men should do to you, do ye even so to them."

  Our own nature illustrates this truth, for as the Apostle Paul says, "Much more, those members of the body which seem to be more feeble are necessary; and those members of the body which we think to be less honorable, upon these we bestow more abundant honor; and our uncomely parts have more abundant comeliness." (1 Corinthians 12:22) We have no need to cover our face; eyes, nose, and mouth; for as our most honorable members, they do not require it. However, we cover those members of which we are ashamed. Likewise, we cover the blemishes and infirmities we find in our neighbor. In this way we serve him and help him. Unless what our neighbor has done is notoriously evil, it is a virtue for us to put the best construction on all we hear. There are enough poison tongues busy trying to put the worst construction on things. This is especially true of any fault found with a preacher, or anything by which the gospel might be put in a bad light or discredited.
Therefore, this commandment comprehends a multitude of good works, all of which are acceptable to God. Yet the blind world, with all of its false saints, fails to recognize them. For in temporal matters, the tongue is feeble, yet capable of both great good and great evil.

[The above is based upon, and closely follows, Martin Luther's explanation of the Eighth Commandment]