The Ten Commandments begin with two commandments against idolatry. Then come commandments 3 — 10. Why this order? It is because the fundamental problem in law breaking is always idolatry. In other words, we never break
commandments 3 — 10 without first breaking 1 and 2.
We will either worship God or other things. We cannot eliminate God without creating God substitutes. Something will capture our hearts and imaginations and be the most important thing to us — our ultimate concern, value, and allegiance. So every personality, community, and thought form will be based on either God himself or on some God substitute, an idol.
This means that idolatry is ultimately the reason for all wrongdoing. Why do we ever lie or steal or covet? Of course, the general answer is “because we are weak and sinful,” but the specific answer is always because there is something besides Jesus Christ that we feel we must have to be happy, something that is more important to our hearts than God, something that is enslaving our hearts through inordinate desires. All our failures to trust God wholly or to live rightly are rooted in idolatry — something we make more important than God.
…Therefore, in sin we are always forgetting what God has done for us and instead are being moved by some idol. That is precisely what happened to the Israelites in the desert.
Note also that God first rescues the people from Egypt, and then he gives them the Ten Commandments. Keeping the Ten Commandments is not what saved them; God had already done that. God did not first give the Law and then deliver the people – first he delivered his people, and then he gave them the Law. Thus we are not saved by the Law, but saved for the Law. The Law is how we regulate our love relationship with God, not the way we merit the relationship. We are saved by faith in Christ alone.
– Tim Keller