The tongue is such a powerful force and capable of such evil (James 3:6-8) that two commandments are required concerning it. One protects God's name and another our neighbor's (Exodus 20:16).

1. The Scope Of The Ninth Commandment
Synecdoche being used in the giving of the law causes us to understand that this commandment forbids all manner of falsehoods. Slander is simple the worst in a class of sins (Deuteronomy 19:16-20). The ninth commandment actually forbids all lying, fraud, false witness, deceit, excuses, hypocrisy, flattery, broken promises, duplicity… etc. True love will cause us to be truthful in all our words and dealings.

2. The Father Of Lies
Liars are following the example of Satan. He was the first liar (John 8:44, Genesis 3:1-5), and the inspiration behind many subsequent lies (Acts 5:3). He is also a slanderer and an accuser (Job 1:9-12, Revelation 12:9-10). There is not one atom of love in his heart for others. His whole kingdom is built on deception and deceit (Matthew 24:24, II Thessalonians 2:8-10). May God give us grace to be like Christ the "true witness" (Revelation 3:14).

3. The Seriousness Of Our Speech
A. Our words can harm others. The man who says "its only words" is a fool. Words have hurt more people than "sticks and stones" (Proverbs 18:21, 10:11, 26:18-19). Particularly dangerous is the man who lies in God's name about God's Word.
B. God hates lying words (Proverbs 12:22).
C.God will judge every word of man (Matthew 12:36-37).
D. Lying leads to other sins. One lie always demands another lie. Lying also is the ploy that men trust in to protect them from the penalty of other sins. What man would cheat, steal, commit adultery, or murder without feeling that if questioned he could take shelter in lying. A man who is always truthful is delivered from a host of sins.
E. Loss of credibility is the inevitable result of dishonest speech. No liar remains successful in his deception. The liar loses his right to be trusted and sooner or later men suspect his every word.
F. Loss of Peace - Sin destroys our inner peace and good conscience. The liar soon forgets to whom he said what and comes to fear detection.
G. Hell is the destiny of all liars not washed and changed by the Lord Jesus Christ (Revelation 21:8, Psalm 15:1-3).

4. Help To Truthfulness
A. Let us instill into our children a strict regard for truth. Our personal example of honesty is necessary for our instruction to be successful.
B. May we remember God's omniscience. No one can successfully deceive God and no lie is hid from Him.
C. We must always be scrupulous with our words and promises. Let us carefully consider promises and commitments before we make them. Once we promise let us carefully keep our promise. We live in an age when men's words mean nothing.
D. Let us use great care in speaking of others. Only that which is strictly true should be spoken. Beyond that we should keep the golden rule. Negative things about our neighbors should only be repeated if there is a good reason for it (I Peter 4:8). Ephesians 4:15 sums up our duty in this regard.
E. Christians should refuse to listen to gossip and the unnecessary exposure of our neighbors' faults. If it is wrong to gossip it is wrong to encourage it by listening. Such tattlers should rather be rebuked (Proverbs 25:23).

Conclusion - God's law ever reveals His holy standard and reveals to man his lost condition. God could easily condemn us on the basis of our speech. Conviction of this shows men their need of Christ (Matthew 12:34-37). Likewise we that are already saved use the law as our guide. We learn to be careful in the use of our tongue (Psalm 19:14).