(Emphases are my own.)
–From The Heidelberg Catchism (H/T RCA.org)
Lord’s Day 43.
Q. What is the aim of the ninth commandment?
A. That I never give false testimony against anyone, twist no one’s words, not gossip or slander, nor join in condemning anyone rashly or without a hearing.
Rather, in court and everywhere else, I should avoid lying and deceit of every kind; these are the very devices the devil uses, and they would call down on me God’s intense wrath. I should love the truth, speak it candidly, and openly acknowledge it. And I should do what I can to guard and advance my neighbor’s good name.
–From The Westminster Larger Catechism (H/T OPC.org)
Q. 143. Which is the ninth commandment?
A. The ninth commandment is, Thou shalt not bear false witness against thy neighbour.
Q. 144. What are the duties required in the ninth commandment?
A. The duties required in the ninth commandment are, the preserving and promoting of truth between man and man, and the good name of our neighbor, as well as our own; appearing and standing for the truth; and from the heart, sincerely, freely, clearly, and fully, speaking the truth, and only the truth, in matters of judgment and justice, and in all other things whatsoever; a charitable esteem of our neighbors; loving, desiring, and rejoicing in their good name; sorrowing for and covering of their infirmities; freely acknowledging of their gifts and graces, defending their innocency; a ready receiving of a good report, and unwillingness to admit of an evil report, concerning them; discouraging talebearers, flatterers, and slanderers; love and care of our own good name, and defending it when need requireth; keeping of lawful promises; studying and practicing of whatsoever things are true, honest, lovely, and of good report.
Q. 145. What are the sins forbidden in the ninth commandment?
A. The sins forbidden in the ninth commandment are, all prejudicing the truth, and the good name of our neighbors, as well as our own, especially in public judicature; giving false evidence, suborning false witnesses, wittingly appearing and pleading for an evil cause, outfacing and overbearing the truth; passing unjust sentence, calling evil good, and good evil; rewarding the wicked according to the work of the righteous, and the righteous according to the work of the wicked; forgery, concealing the truth, undue silence in a just cause, and holding our peace when iniquity calleth for either a reproof from ourselves, or complaint to others; speaking the truth unseasonably, or maliciously to a wrong end, or perverting it to a wrong meaning, or in doubtful or equivocal expressions, to the prejudice of the truth or justice; speaking untruth, lying, slandering, backbiting, detracting, talebearing, whispering, scoffing, reviling, rash, harsh, and partial censuring; misconstructing intentions, words, and actions; flattering, vainglorious boasting, thinking or speaking too highly or too meanly of ourselves or others; denying the gifts and graces of God; aggravating smaller faults; hiding, excusing, or extenuating of sins, when called to a free confession; unnecessary discovering of infirmities; raising false rumors, receiving and countenancing evil reports, and stopping our ears against just defense; evil suspicion; envying or grieving at the deserved credit of any; endeavoring or desiring to impair it, rejoicing in their disgrace and infamy; scornful contempt, fond admiration; breach of lawful promises; neglecting such things as are of good report, and practicing, or not avoiding ourselves, or not hindering what we can in others, such things as procure an ill name.
–Thorough exposition of 9th commandment and its implications by Phil Johnson of Grace Community Church: ‘Putting Away Lying, Speak the Truth’.
Thou shalt not bear false witness against thy neighbor.
 Over and above our own body, spouse, and temporal possessions, we have yet another treasure, namely, honor and good report [the illustrious testimony of an upright and unsullied name and reputation], with which we cannot dispense. For it is intolerable to live among men in open shame and general contempt.  Therefore God wishes the reputation, good name, and upright character of our neighbor to be taken away or diminished as little as his money and possessions, that every one may stand in his integrity before wife, children, servants, and neighbors.  And in the first place, we take the plainest meaning of this commandment according to the words (Thou shalt not bear false witness), as pertaining to the public courts of justice, where a poor innocent man is accused and oppressed by false witnesses in order to be punished in his body, property, or honor.
 Now, this appears as if it were of little concern to us at present; but with the Jews it was quite a common and ordinary matter. For the people were organized under an excellent and regular government; and where there is still such a government, instances of this sin will not be wanting. The cause of it is that where judges, burgomasters, princes, or others in authority sit in judgment, things never fail to go according to the course of the world; namely, men do not like to offend anybody, flatter, and speak to gain favor, money, prospects, or friendship; and in consequence a poor man and his cause must be oppressed, denounced as wrong, and suffer punishment. And it is a common calamity in the world that in courts of justice there seldom preside godly men.
 For to be a judge requires above all things a godly man, and not only a godly, but also a wise, modest, yea, a brave and hold man; likewise, to be a witness requires a fearless and especially a godly man. For a person who is to judge all matters rightly and carry them through with his decision will often offend good friends, relatives, neighbors, and the rich and powerful, who can greatly serve or 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 first of all that every one shall help his neighbor to secure his rights, and not allow them to be hindered or twisted, but shall promote and strictly maintain them, no matter whether he be judge or witness, and let it pertain to whatsoever it will.  And especially is a goal set up here for our jurists that they be careful to deal truly and uprightly with every case, allowing right to remain right, and, on the other hand, not perverting anything [by their tricks and technical points turning black into white and making wrong out to be right], nor glossing it over or keeping silent concerning it, irrespective of a person’s money, possession, honor, or power. This is one part and the plainest sense of this commandment concerning all that takes place in court.
 Next, it extends very much further, if we are to apply it to spiritual jurisdiction or administration; here it is a common occurrence that every one bears false witness against his neighbor. For wherever there are godly preachers and Christians, they must bear the sentence before the world that they are called heretics, apostates, yea, seditious and desperately wicked miscreants. Besides, the Word of God must suffer in the most shameful and malicious manner, being persecuted, blasphemed, contradicted, perverted, and falsely cited and interpreted. But let this pass; for it is the way of the blind world that she condemns and persecutes the truth and the children of God, and yet esteems it no sin.
 In the third place, what concerns us all, this commandment forbids all sins of the tongue whereby we may injure or approach too closely to our neighbor. For to bear false witness is nothing else than a work of the tongue. Now, whatever is done with the tongue against a fellow-man God would have prohibited, whether it be false preachers with their doctrine and blasphemy, false judges and witnesses with their verdict, or outside of court by lying and evil-speaking.  Here belongs particularly the detestable, shameful vice of speaking behind a person’s back and slandering, to which the devil spurs us on, and of which there would be much to be said. For it is a common evil plague that every one prefers hearing evil to hearing good of his neighbor; and although we ourselves are so bad that we cannot suffer that any one should say anything bad about us, but every one would much rather that all the world should speak of him in terms of gold, yet we cannot bear that the best is spoken about others.
Therefore, to avoid this vice we should note that  no one is allowed publicly to judge and reprove his neighbor, although he may see him sin, unless he have a command to judge and to reprove.  For there is a great difference between these two things, judging sin and knowing sin. You may indeed know it, but you are not to judge it. I can indeed see and hear that my neighbor sins, but I have no command to report it to others. Now, if I rush in, judging and passing sentence, I fall into a sin which is greater than his. But if you know it, do nothing else than turn your ears into a grave and cover it, until you are appointed to be judge and to punish by virtue of your office.
 Those, then, are called slanderers who are not content with knowing a thing, but proceed to assume jurisdiction, and when they know a slight offense of another, carry it into every corner, and are delighted and tickled that they can stir up another’s displeasure [baseness], as swine roll themselves in the dirt and root in it with the snout.  This is nothing else than meddling with the judgment and office of God, and pronouncing sentence and punishment with the most severe verdict. For no judge can punish to a higher degree nor go farther than to say: “He is a thief, a murderer, a traitor,” etc. Therefore, whoever presumes to say the same of his neighbor goes just as far as the emperor and all governments. For although you do not wield the sword, you employ your poisonous tongue to the shame and hurt of your neighbor.
 God therefore would have it prohibited, that any one speak evil of another even though he be guilty, and the latter know it right well; much less if he do not know it, and have it only from hearsay. But you say:  Shall I not say it if it be the truth? Answer: Why do you not make accusation to regular judges? Ah, I cannot prove it publicly, and hence I might be silenced and turned away in a harsh manner [incur the penalty of a false accusation]. “Ah, indeed, do you smell the roast?” If you do not trust yourself to stand before the proper authorities and to make answer, then hold your tongue. But if you know it, know it for yourself and not for another. For if you tell it to others, although it be true, you will appear as a liar, because you cannot prove it, and you are, besides, acting like a knave. For we ought never to deprive any one of his honor or good name unless it be first taken away from him publicly.
 False witness, then, is everything which cannot be properly proved.  Therefore, what is not manifest upon sufficient evidence no one shall make public or declare for truth; and, in short, whatever is secret should be allowed to remain secret, or, at any rate, should be secretly reproved, as we shall hear.  Therefore, if you encounter an idle tongue which betrays and slanders some one, contradict such a one promptly to his face, that he may blush; thus many a one will hold his tongue who else would bring some poor man into bad repute, from which he would not easily extricate himself. For honor and a good name are easily taken away, but not easily restored.
 Thus you see that it is summarily forbidden to speak any evil of our neighbor, however, the civil government, preachers, father and mother excepted, on the understanding that this commandment does not allow evil to go unpunished. Now, as according to the Fifth Commandment no one is to be injured in body, and yet Master Hannes [the executioner] is excepted, who by virtue of his office does his neighbor no good, but only evil and harm, and nevertheless does not sin against God’s commandment, because God has on His own account instituted that office; for He has reserved punishment for His own good pleasure, as He threatens in the First Commandment,–just so also, although no one has a right in his own person to judge and condemn anybody, yet if they to whose office it belongs fail to do it, they sin as well as he who would do so of his own accord, without such office. For here necessity requires one to speak of the evil, to prefer charges, to investigate and testify;  and it is not different from the case of a physician who is sometimes compelled to examine and handle the patient whom he is to cure in secret parts. Just so governments, father and mother, brothers and sisters, and other good friends, are under obligation to each other to reprove evil wherever it is needful and profitable.
 But the true way in this matter would be to observe the order according to the Gospel, Matt. 18, 15, where Christ says: If thy brother shall trespass against thee, go and tell him his fault between thee and him alone. Here you have a precious and excellent teaching for governing well the tongue, which is to be carefully observed against this detestable misuse. Let this, then, be your rule, that you do not too readily spread evil concerning your neighbor and slander him to others, but admonish him privately that he may amend [his life]. Likewise, also, if some one report to you what this or that one has done, teach him, too, to go and admonish him personally, if he have seen it himself; but if not, that he hold his tongue.
The same you can learn also from the daily government of the household.
 The same you can learn also from the daily government of the household. For when the master of the house sees that the servant does not do what he ought, he admonishes him personally. But if he were so foolish as to let the servant sit at home, and went on the streets to complain of him to his neighbors, he would no doubt be told: “You fool, what does that concern us?  Why do you not tell it to him?” Behold, that would be acting quite brotherly, so that the evil would be stayed, and your neighbor would retain his honor. As Christ also says in the same place: If he hear thee, thou hast gained thy brother. Then you have done a great and excellent work; for do you think it is a 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 he whom it concerns is always to be treated with personally, and not to be spoken of without his knowledge.  But if that do not avail, then bring it publicly before the community, whether before the civil or the ecclesiastical tribunal. For then you do not stand alone, but you have those witnesses with you by whom you can convict the guilty one, relying on whom the judge can pronounce sentence and punish. This is the right and regular course for checking and reforming a wicked person.  But if we gossip about another in all corners, and stir the filth, no one will be reformed, and afterwards when we are to stand up and bear witness, we deny having said so.  Therefore it would serve such tongues right if their itch for slander were 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 nor shun the day and the light.
For when a matter is public in the light of day, there can be no slandering or false judging or testifying. … For where the sin is public, the reproof also must be public, that every one may learn to guard against it.
 All this has been said regarding secret sins. But where the sin is quite public so that the judge and everybody know it, you can without any sin avoid him and let him go, because he has brought himself into disgrace, and you may also publicly testify concerning him. For when a matter is public in the light of day, there can be no slandering or false judging or testifying; as, when we now reprove the Pope with his doctrine, which is publicly set forth in books and proclaimed in all the world. For where the sin is public, the reproof also must be public, that every one may learn to guard against it.
 Thus we have now the sum and general understanding of this commandment, to wit, that no one do any injury with the tongue to his neighbor, whether friend or foe, nor speak evil of him, no matter whether it be true or false, unless it be done by commandment or for his reformation, but that every one employ his tongue and make it serve for the best of every one else, to cover up his neighbor’s sins and infirmities, excuse them, palliate and garnish them with his own reputation.  The chief reason for this should be the one which Christ alleges in the Gospel, in which He comprehends all commandments respecting our neighbor, Matt. 7, 12: Whatsoever ye would that men should do to you, do ye even so to them.
 Even nature teaches the same thing in our own bodies, as St. Paul says, 1 Cor. 12, 22: 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. No one covers his face, eyes, nose, and mouth, for they, being in themselves the most honorable members which we have, do not require it. But the most infirm members, of which we are ashamed, we cover with all diligence; hands, eyes, and the whole body must help to cover and conceal them.  Thus also among ourselves should we adorn whatever blemishes and infirmities we find in our neighbor, and serve and help him to promote his honor to the best of our ability, and, on the other hand, prevent whatever may be discreditable to him.  And it is especially an excellent and noble virtue for one always to explain advantageously and put the best construction upon all he may hear of his neighbor (if it be not notoriously evil), or at any rate to condone it over and against the poisonous tongues that are busy wherever they can pry out and discover something to blame in a neighbor, and that explain and pervert it in the worst way; as is done now especially with the precious Word of God and its preachers.
 There are comprehended therefore in this commandment quite a multitude of good works which please God most highly, and bring abundant good and blessing, if only the blind world and the false saints would recognize them. For there is nothing on or in entire man which can do both greater and more extensive good or harm in spiritual and in temporal matters than the tongue, though it is the least and feeblest member.