You shalt have no other gods before Me.
  That is: You shall have Me alone as your God. You shall not  make for yourself any graven image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. You shall not bow down to them and serve them.
  Briefly stated, this means that We should fear, love, and trust in God above all things. If you truly have faith in Christ, then you have the True God. On the other hand, if you do not have faith in Christ, then you do not have the true God; for faith and God go together. A god is that from which we are to expect all good and to which we are to take refuge in all distress. For that reason, that in which you place your trust is your god.
 Therefore, the intent of this commandment is to require true faith in Christ. Faith that clings to the Triune God alone, looks to Him alone for every blessing, and trusts in Him alone for help in every need.
 Far too many people think God is pleased with them just because everything is going well in their lives, or because they have a lot of money and possessions. Earthly wealth gives men a false sense of security. Such men have a false faith and, therefore, a false god -- Mammon by name. Their trust is not in the True God, but in their money and possessions. Those are the things on which they set their heart. This is also the most common idol on earth. On the other hand, those who lack money often have doubts about their salvation, or are despondent and bitter, as if they had no God. This glorification of money and the things that it can buy sticks and clings to our nature,even to the grave.
 Whoever trusts and boasts that he possesses great skill, prudence, power, favor, friendship, and honor also has a god, but not the true and only God. This appears again when you notice how presumptuous, secure, and proud people are because of such possessions, and how despondent when they no longer exist or are withdrawn. Therefore I repeat that the chief explanation of this point is that to have a god is to have something in which the heart entirely trusts.
 Since a person's god is that in whom he trusts, devotion to the saints, as taught by the Papacy, is false worship. Under the Papacy, if any one had a toothache he fasted in honor of St Apollonia. If he was afraid of fire he chose St. Lawrence as his helper in need. If he dreaded pestilence he made a vow to St. Sebastian or Rochio. There were a countless number of such abominations, where every one selected his own saint, worshiped him, and called to him for help in distress. Such false worship belongs in the same class as that of sorcerers and magicians, whose idolatry is most gross, and who make a covenant with the devil in the hope that he will give them money, help them in love-affairs, preserve their cattle, or restore lost possessions, etc.. All such people fail to place their trust in the true God. They neither seek His mercy nor expect anything good from Him.
 Thus this commandment requires us to place our confidence in God alone and trust in no one else. We cannot take God with our hands. We take Him with our heart by clinging to Him in faith, trusting in Him, and looking to Him in every need. For this reason, He wishes to turn us away from everything else that exists outside of Him, and to draw nigh unto Him, for He is the only eternal good. He alone can give you heaven, and pour out upon you all good things.
 Only when we so place our confidence in God, trusting in Christ and Christ alone for salvation are we able to honor or worship God in a way that is pleasing to Him. He commands such worship under penalty of eternal wrath. Rather than be torn from Him, we should risk and be willing to disregard everything on earth. How far short the world falls on the matter. You can easily see how the world practices only false worship and idolatry, for every one has set up as his special god those things he looks to for blessings, help, and comfort. We can see this illustrated in the lives of the heathen. Those who put their trust in power and dominion elevated Jupiter as the supreme god. Those who were bent upon riches, happiness, pleasure, and a life of ease worshiped Hercules, Mercury, Venus or others. Women with child worshiped Diana or Lucina, and so on. They made a god of those things they trusted in for comfort, so that in which they put their faith became their god. They are often sincere, but they are misguided. They have a false faith, for they have not trusted in the One True God. Their self-invented notions and dreams of God became their idol, and they put their trust in that which is altogether nothing. Likewise, those who trust in science deny God, or invent a god who used evolution. All idolatry is the same, for idolatry does not consist only of erecting an image and worshiping it, but of placing ones trust in something other than God. The lost seek consolation from creatures, saints, or devils, but they do not care for God, look to Him for help, or believe that all good they experience comes from Him.
 There is another type of false worship prevalent in the world. It is a false worship that concerns the conscience alone, a false worship that seeks help, consolation, and salvation through works. Those who practice this false worship would wrest heaven from God by their works. They seek to put God in their debt by tithing, praying, fasting, and attending church. They trust in such things and are unwilling to receive anything from God as a gift. They would have God serve them, as if He were their debtor and they His landlord. Such devotion to works is just another way in which men attempt to reduce God to an idol who will do their bidding, while striving to make themselves His master.
 The important thing to stress when teaching this commandment is that we are to trust in God alone, look to Him alone, and expect only good from Him as we would from a loving Father. We should understand that it is He who gives us our body, our life, our food, our drink, our nourishment, our health, our protection, our peace, and all necessities both temporal and eternal. He also preserves us from misfortune, and if any evil befalls us He delivers and rescues us. All the good that we receive comes to us from Him alone, and it is by His grace that we are delivered from evil. For that reason, it is fitting that our word, "God" comes from the word, "good" -- elegantly expressing the fact that He is the only and eternal source of all good; the source of every blessing.
 For even though we experience much good from men, whatever we receive by God's command or arrangement is received from God. For our parents, our rulers, and our neighbors have received from God the command to do us good, therefore any good we receive at their hands does not come from them but through them from God. Men are only the hands, channels, and means whereby God gives all things.  He gives to the mother breasts and milk to offer to her child and corn and all manner of produce from the earth for nourishment. All of those things are blessings of God that no man could produce in and of himself.
 Therefore, that all good might be acknowledged as God's gift and thanks rendered to Him for it, we should never presume to take or give anything except as God has commanded. For that reason, we should never refuse what is given to us according to God's command, nor should we seek to gain by ways and means other than those God has commanded. To do so would not be receiving from God, but seeking of ourselves.
 Everyone should realize the importance of this commandment and take it seriously. Those who examine their own heart, being honest with themselves, will know if their heart clings to God alone or not. If you look to Him alone for salvation, believing, even in want and distress, that He will make all things work together for your good, then you have the only true God. If, on the other hand, your heart looks to Mary, the Saints, or any other thing for help while fearing that God wants your harm, then you have an idol, a false god.
 In order to make it clear that He will not have this commandment thrown to the winds but will most strictly enforce it, God has attached to it a terrible threat, and a beautiful, comforting promise  both of which are to be impressed upon young people, that they may take it seriously and remember it.

 [Exposition of the Appendix to the First Commandment.]
 For I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate Me; and showing mercy unto thousands of them that love Me and keep My commandments.
 Although these words relate to all the commandments (as we shall hereafter learn), they are joined to this chief commandment because it is vitally important for us to have the right foundation. If our faith rests on the right foundation everything else will be right. On the other hand, if it rests on a wrong foundation then everything else will be wrong. These words impress upon us God's wrath against all who do not trust in Him and His grace which is freely extended to all who trust in His mercy. The fact that His anger does not cease until the fourth generation -- while His blessing and goodness extend to many thousands -- should be a warning to all who lapse into a false security and commit the final resting place of their soul to chance. It should likewise be a warning to those brutes who think that their way of living does not make any difference. If men turn from God, He will have vengeance and will not cease to be angry until the fourth generation, even if that means their utter extermination. Therefore, He is to be feared and not to be detested.
 History is filled with demonstrations of this truth. The Bible and everyday experience provide us with many examples. From the beginning God has uprooted all idolatry. His judgement upon those who reject Him has fallen upon both heathen and Jews, and false worship continues to fall before the power of His Word. For that reason, all who remain therein must finally perish. The rich, powerful, and proud worldlings who are secure in their riches and care less if God is angry at them shall be destroyed with all in which they trusted. They shall suffer the same fate as all others who have thought themselves secure or powerful, yet died without Christ.
 Since God does not always destroy such hardened souls immediately, they often assume that God is ignorant or does not care about such matters. Yet He will deal them a smashing blow that will extend even unto their children's children; so that every one may take note and see that this is no joke to Him. Those who persist in their defiance and pride are the ones He is speaking of when He says, "Who hate Me". They will not listen to what is preached or said to them. Nor will they amend their ways before the punishment begins. When they are reproved they become angry and hateful, making themselves even more deserving of God's wrath, as we see demonstrated daily in the behavior of  bishops and princes.
 However, as terrible as these threatenings are, God's grace is greater. Those who cling to God alone have the sure promise of His mercy in Christ. If the warning of His wrath is powerful, the promise of His mercy is even more powerful. Those who cling to God alone can be sure that He will show them mercy, and His blessing will extend to their children and children's children even to the thousandth generation and beyond. Such a great promise ought to impel us to trust ourselves fully to God's mercy and grace. Who could reject His offer knowing that in Christ we have all temporal and eternal good?
 Everyone should take this promise seriously, and realize that it is not merely the word of man but the sure Word of God. Every blessing of heaven belongs to those who heed God's commandments, admit their sinfulness, and place their trust in His promise of mercy. In contrast, those who refuse to hear God's commandments shut out the truth and deny their need of His mercy. On the day of judgement they will receive only wrath and torment. Therefore, it is a question either of eternal blessing, happiness, and salvation or of eternal wrath, misery, and woe. What great comfort it is to know that God so kindly promises to be ours with every blessing, and to protect and help us in all need.
 But, alas! The world believes nothing of this, nor regards it as God's Word, because it sees those who profess to trust in God suffer poverty or find it difficult to make ends meet; while those who serve Mammon have power, favor, honor, possessions, and every comfort in the eyes of the world. These words teach us that we should not base our judgements on such outward appearances. We must realize that God's Word does not lie or deceive, but must come true.

 What is gained in the long run by those who devote all of their energy to amassing wealth? Can you tell me of any who have not wasted their toil and labor? What have they finally attained? We often hear of men who have gained great wealth yet have never found happiness. In the end all that they have accumulated is dispersed and scattered, never reaching the third generation. There are many examples of this in history. Older people are often personally aware of such a situation. Ponder the folly of such people and put God first in your life.
 Saul was a great king who had everything going for him. He was a godly man who had been chosen by God to rule Israel. However, when he let his heart decline from God and put his trust in his crown and power, he lost everything, including his own life, so that none of his children remained.
 David, on the other hand, was a poor, despised man, hunted down and chased so that he was in constant danger; yet he remained and became king in spite of Saul -- for God's Word had to come true, since God cannot lie or deceive. Therefore, do not allow the devil and the world to deceive you with their show, which indeed remains for a time, but finally is nothing.
 From the First Commandment we learn that our faith must be in God alone because He will not tolerate those who put their faith in another. We should commit our lives to Him and trust in Him for every blessing while using the things which He has given us to provide for our needs, as a shoemaker might use an awl, without allowing any of those things to become our idol. Let this suffice with respect to the First Commandment, which we have had to explain at length, since it is of chief importance because, as before said, where the heart is rightly disposed toward God and this commandment is observed, all the others follow.

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