Exodus 20:3-6 -- God must be your God.

Exodus 20:3 Do not have other gods besides Me.

This is pretty self explanatory.  The Lord God must be the only God we follow after.  It leads directly into the second commandment:

Exodus 20:4 Do not make an idol for yourself, whether in the shape of anything in the heavens above or on the earth below or in the waters under the earth. 5 You must not bow down to them or worship them; for I, the Lord your God, am a jealous God, punishing the children for the fathers’ sin, to the third and fourth generations of those who hate Me, 6 but showing faithful love to a thousand generations of those who love Me and keep My commands.

If we keep God first in our hears and minds, we will not make idols of anything to take his place.  Here God gives a specific punishment and reward to breaking and keeping this law.  If you bow to another idol, whether physically or mentally, you will be punished, your children will be punished, your grandchildren will be punished and so on.  Those who keep the command will be blessed, with their children and grandchildren and so on.

God is jealous.  He wants all of our hearts and minds.  Not part.  Not this little piece over here, he wants it all.

Leviticus 19:4 Do not turn to idols or make cast images of gods for yourselves; I am Yahweh your God.

Here the command is repeated, this time without the consequence.  He says do not make idols because I am Yahweh your God.  I am God.  That is all the excuse He needs to want your full heart and mind and obedience.

Again there is greater incite to this command:

Deuteronomy 4:15 “For your own good, be extremely careful—because you did not see any form on the day the Lord spoke to you out of the fire at Horeb— 16 not to act corruptly and make an idol for yourselves in the shape of any figure: a male or female form, 17 or the form of any beast on the earth, any winged creature that flies in the sky, 18 any creature that crawls on the ground, or any fish in the waters under the earth. 19 When you look to the heavens and see the sun, moon, and stars—all the array of heaven—do not be led astray to bow down and worship them. The Lord your God has provided them for all people everywhere under heaven.20 But the Lord selected you and brought you out of Egypt’s iron furnace to be a people for His inheritance, as you are today.

The Lord chose us to be his.  This alone should keep us from making idols.  The Lord said that when he appeared to the Israelites, he did not show them His form.  He was just a smoke cloud to the people.  God did not reveal his true form, so the people cannot make a form and call it Him.  We were made in God's image, but we did not create God, so we cannot conform God to an image we know.  He has chosen not to reveal that to us at this present time, so we should not attempt to create an image of Him.  He also says we should not worship the sun, moon and stars.  God placed them on earth for all to enjoy.  They are His creation.  We should not worship something that was created, but worship the one that did the creating.

Time after time though out scripture, we see where Israel turns from God and becomes an adulterous nation by putting other gods in place of the one true God.

Exodus 32:7 The Lord spoke to Moses: “Go down at once! For your people you brought up from the land of Egypt have acted corruptly. 8 They have quickly turned from the way I commanded them; they have made for themselves an image of a calf. They have bowed down to it, sacrificed to it, and said, ‘Israel, this is your God, who brought you up from the land of Egypt.’”9 The Lord also said to Moses: “I have seen this people, and they are indeed a stiff-necked people. 10 Now leave Me alone, so that My anger can burn against them and I can destroy them. Then I will make you into a great nation.”

This is probably one of the most recognized stories of idolatry among the Israelites.  Moses did not return when the people expected.  The wanted something to worship and Aaron make them a golden calf.  Aaron... Moses's brother.  The one who stood beside Moses and saw how all the plagues happened, crated the golden calf for the people to worship.  God wanted to completely destroy the people for doing this.  Moses interceded and God chose not to completely destroy them.

2 Kings 21:10 The Lord spoke through His servants the prophets, saying, 11 “Since Manasseh king of Judah has committed all these detestable things—greater evil than the Amorites who preceded him had done—and by means of his idols has also caused Judah to sin, 12 this is what the Lord God of Israel says: ‘I am about to bring such disaster on Jerusalem and Judah that everyone who hears about it will shudder. 13 I will stretch over Jerusalem the measuring line used on Samaria and the mason’s level used on the house of Ahab, and I will wipe Jerusalem clean as one wipes a bowl—wiping it and turning it upside down. 14 I will abandon the remnant of My inheritance and hand them over to their enemies. They will become plunder and spoil to all their enemies, 15 because they have done what is evil in My sight and have provoked Me from the day their ancestors came out of Egypt until today.’”

The punishment here was quit severe.  Jerusalem was wiped clean.  All the was detestable and dirty was taken away.  Manasseh's son was just like his father and he was killed in his own home.  The above are just a few examples of Israel's disobedience.

God takes his place in our lives seriously.  He wants it all.  He does not want us to follow after anyone or anything but Him. That was true for the Israelites and that is true for us today.  God's punishment for not obeying and following him completely are always severe.  Complete destruction.  The Israelites who followed after idols gained God's wrath and vengeance and were ultimately destroyed, while today, if we do not obey God fully, while we may not suffer now on earth, we will suffer for eternity.

1 Corinthians 6:8 Instead, you act unjustly and cheat—and you do this to believers! 9 Don’t you know that the unrighteous will not inherit God’s kingdom? Do not be deceived: No sexually immoral people, idolaters, adulterers, or anyone practicing homosexuality, 10 no thieves, greedy people, drunkards, verbally abusive people, or swindlers will inherit God’s kingdom.