Briefly stated that means that we should fear and love God that we may not curse, swear, use witchcraft, lie, or deceive by His name, but call upon it in every trouble, pray, praise, and give thanks.
The first commandment taught us that our faith must be in God alone,
and because Christ is God only those who trust in Christ can truly obey
that commandment. This commandment teaches us that when we truly trust
in God, we will honor Him with our lips and tongue. In other words, if
our heart is right our words will also be right for our words reveal what
is in our heart. Therefore, true obedience to this, and every other commandment,
must stem from faith in Christ, not from faith in our own ability and works.
The second commandment forbids the misuse of God's name. Such misuse occurs whenever God's name is used to cover up, promote, or endorse evil. We are not to use God's name for purposes of falsehood or wrong of any kind. God's name must never be used in a careless, vindictive, or thoughtless way; nor is it to be appealed to as witness to what is not true, as is done when those who swear in court to tell the truth lie against each other.
While it is impossible to enumerate all of the ways in which God's name is misused, such misuse happens often in everyday business, in the marketplace as well as in court. Whenever money, property, or honor is at stake, men make false oaths in God's name. It is also true in marriage, when those who have pledged themselves to each other before God break that pledge. However, the greatest abuse occurs in spiritual matters, whenever false preachers offer their lies and ideology as God's Word.
Those who lack the fear of God love to use God's name to cover up their own sins. They put on a pretty show, whether in business or in matters of faith and doctrine. Among them also belong the foul- mouthed, those who blaspheme God, and those who slander His Word or consign it to the devil. They all claim to be right and disgrace God's name without fear. Such men are not for us, but for the hangman to discipline.
We should realize the importance of this commandment, take it seriously, and dread every misuse of the Holy Name. Learn to diligently guard against abuse of God's name, and regard such use of it as a horrendous sin. Lying and deceiving is a great sin in itself, but it becomes much greater when we when we use God's name to excuse it, cover it up, or justify it. By using God's name as a cloak for sin, a single lie results in many lies.
God did not add a serious threat to this commandment only to take it lightly. "The Lord will not hold him guiltless that taketh His name in vain." He is not going to condone such sin or allow it to pass unpunished. All who turn their heart from God will face His judgement, for He is not going to allow His name to be used as cover for a lie. Those who put on an outward show of piety to cover their dishonesty and immorality shall not escape. However, the sad tragedy is that so many abuse the name of God to cover lies and wickedness; while so few trust in God alone for salvation.
We all, by nature, see the need to cover up our sin, adorn it, and hide it. There are few so depraved that they would boast of a sin, and none who will admit all of the wicked things they have done, much less boast about them. We cover up wickedness because we know that it deserves condemnation. We act by stealth and hide the truth because we do not want people to know what we have done. However, when God is dragged into the affair, and His name is used to make the villainy look like godliness and the shame look like honor, the sin is made greater not less. There is only one thing that will truly cover sin and that is the shed blood of Jesus Christ. Yet the way of this world is to trust in deception rather than Christ, and in lies rather than in God's mercy (John 1:47). This is the great delusion that covers the world like a flood. And the fruit of that delusion is only what we deserve: pestilence, war, famine, conflagration, floods, wayward wives, children, servants, and all sorts of defilement. Where else would so much misery come from? It is only by the mercy of God that the earth still supports us.
Therefore, our young people should be trained to take this commandment seriously, and whenever they transgress they need to be disciplined. Explain to them why it is important to be respectful of God's name. Use punishment when necessary, but bring them up not only with punishment, but also in the reverence and fear of God.
In summary, God's name is taken in vain whenever it is used to deceive, curse, swear, conjure, or in connection with any other wickedness.
Having learned what constitutes abuse of God's name, we must learn the right use of God's name. By forbidding the wrong use of His name, God lets us know that there is indeed a right use of His name. He has revealed that name and given it to us, so that it might be of constant use and profit. Since we are forbidden to use the holy name for falsehood or wickedness, it follows that we are only to employ it for truth and good. One example might be when we swear to the truth in court. Another example would be when we use God's name in connection with sound doctrine. God's name is also used aright when it is praised and thanked, or when we cry out to God in time of distress. As it is written, "Call upon Me in the day of trouble: I will deliver thee, and thou shalt glorify Me" (Psalm 50:15). When we pray, "Hallowed be Thy name", we are asking that God's name so be used in a godly and right way (Luke 11:2).
Thus you have the commandment explained. With this understanding, the question of why The Apostle Paul and other men of God often swore, when swearing is prohibited in the Gospel, is easily solved. Many teachers have troubled themselves over that question when the solution is really quite simple. The key issue is whether God's name is being used for good or for evil. We are not to swear when there is no real need to, or in support of falsehood and evil. Yet, when necessary, we should swear in support of truth, justice, and the good of our neighbor. For the right use of God's name is a good work by which God is praised, truth and right are established, falsehood is refuted, peace is made among men, obedience is rendered, and quarrels are settled. God makes a clear distinction between right and wrong, and good and evil. Those who abuse God's name shall not escape punishment, though it may be deferred a long time. All that they may gain by abusing God's name will slip out of their hands, and they will never enjoy it. Many who perjured themselves in their marriage-vows, have never had a happy hour or a healthful day, and perish wretched in body, soul, and possessions.
Therefore, let the children be trained to shun falsehood, and especially to avoid the use of God's name in the support of falsehood. If necessary use warnings, threats, and punishments; but never let them grow up thinking that abuse of God's name of little consequence. If they are allowed to do as they please, no good will come of it. It should be evident that the world is now worse than it has ever been. Abuse of God's name goes unpunished. As a result hearts are hardened, and all teaching or reproof is ignored. Such indifference to God's Word is in itself a punishment for wanton contempt of this commandment.
On the other hand, young people should be constantly guided to honor God's name, to think about His will in everything that may happen to them or come to their notice, and to talk to Him about every problem. That is the right way to honor God's name. We are to look to God's name seeking, guidance, wisdom, understanding, and consolation. By faith our heart first gives God the honor due Him, then our lips honor Him by confession.
When we trust in God's mercy, and commit ourselves to His care, Satan; who would love to bring us into sin, shame, calamity, and trouble; cannot harm us. I have learned by experience that great calamity is quickly averted and removed when we call upon God. Therefore, to thwart the devil, keep your thoughts upon God and His name upon your lips.
Daily commit yourself, your wife, your children, and all that you have to God's care and protection. Start your day with prayer, pray before meals, and end your day with prayer. Learn to say, "Praise the Lord!", or, "Help me dear Lord Jesus!" etc., as the occasion warrants. In that way keep God before your eyes, and His name upon your lips. Such behavior is far more pleasing to God than idolatrous prayer to saints, and the Christ denying sanctity of the monks.
Teach your children by example to fear and honor God. Be consistent
in your observance of the first and second commandments. Let them see how
seriously you take God's Word, and how respectful you are of God's name.
Explain to them why it is important to treat God's name respectfully. Train
them with kindness and patience, for those taught only with blows will
remain godly only as long as the rod is upon their backs. The result will
be only good, and those who learn as a child to honor God in thought, word,
and deed will be a blessing to the entire nation.
When children are trained in this way, God's Word takes root in the heart, and they learn to fear God more than rods and clubs. Present the truth to them simply, that it may penetrate their minds. When we are teaching small children, we must also prattle with them. In this way we prevent abuse and teach the right use of the divine name. A use that consists not only of words, but also of what we do and how we live our life. We know that this is the will of God, and that He will bless the proper use of His name while punishing the abuse.
[The above is based upon, and closely follows, Martin Luther's explanation of the Second Commandment.]