Commandment 10 is great, because it can't be enforced. You can do it, and nobody will ever know!
With murder, there will probably be concrete evidence that it's been done.
With adultery, there will be at least one witness.
Lying is something that is done to another person.
With jealousy, however, people may never know if you've done it. It goes on in the mind instead of being an action. This commandment is very similar to commandment eight: "Don’t take the things that other people have." But in this case, it’s not about the act, but about the heart. God doesn’t only care if you actually take a thing; God’s concerned with your heart. Commandment eight: called people not to violate another person's boundaries; to not hinder their survival. And now commandment ten calls people to refrain from even desiring the things that others have.
This is interesting, because for the most part in the Old Testament, sin is seen as something that a person does. Sin is not something in the heart, but it’s an impulse that you’ve acted upon. That’s why it’s so shocking when Jesus says that if you think angry thoughts about a person, you’re as subject to judgment as if you killed him. Or when he says that if you think impure thoughts about someone, you’ve already committed adultery with that person in your heart. The people would have thought, “No, no, that’s not true. We never sinned," because in the minds of the people, sin was something that you did.
But the words of Jesus show us, and this tenth commandment shows us, that God is concerned with what’s in the heart. God is concerned about your thoughts and your desires. Are you looking at someone else’s things and thinking how great it would be to have those things for yourself? Stop it. Stop it right now. We all know what jealousy can lead to, and especially if you’re living amongst a group of people in the desert, and everyone’s trying to survive, you need to not have these thoughts.
Jealousy is the catalyst that leads to stealing. God calls us to remain grateful for what we've been given, and to kill any thoughts of envy for another person's belongings.