How is it that “coveting” is such a severe sin that it rates inclusion in the Ten Commandments? Coveting does not overtly harm our neighbour. Desiring what my neighbour has does not hurt him, unless coveting gives way to actually acting on that desire, by, for example, violating the commandment against stealing. But coveting is a sin that harms me, even if I never act upon it. Coveting is not a matter of committing external actions, as in other commandments. Rather, it is a condition of the heart.
Coveting is not just as an action but an internal state is the point Jesus makes in the Sermon on the Mount. Not just murder but hatred and contempt violates the commandment against killing. Not just actual fornication but lust violates the commandment against adultery (Matthew 5:21-28).
In fact, the sinful inner state is the prior cause of the other sins. Before you steal, you first covet what belongs to your neighbor. Before you commit adultery, you covet your neighbor’s wife. The original sin of Adam and Eve involved coveting – wanting to be like God (Genesis 3:5).
“Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry.” Colossians 3:5
How is coveting idolatry? To sinfully desire what your neighbor has is to be discontented with what God has given you. It is thus a failure to “fear, love, and trust in God above all things,” Both have to do with the lack of faith.
Graven images can distract us from trusting the true God, and so can our desire for possessions or relationships. But we can trust God to meet all of our needs. And, in fact, He has already given us gifts that are right for us, and He will continue to do so. We do not need to covet what He has done for someone else.
The Law leads us inexorably to the sin within our own heart. And the cure is faith, made possible by the Cross of Jesus Christ, who has atoned for both our external and our internal sins. He offers both forgiveness and an inner transformation wrought by the Holy Spirit that bears fruit in love for both God and our neighbor. Thus the Law leads to the Gospel. As Luther said, if we can just fulfill the first commandment, to have no other gods before the God of the Bible – that is, to have faith in Him–all of the other commandments will take care of themselves.