One theologian has said that "the focus on the heart is not a New Testament innovation. It is also an Old Testament concern." The emphasis on having the right internal motivation did not start with Jesus in the Sermon on the Mount; rather, it has been a part of God's revelation from the very beginning. We see this quite clearly in the tenth commandment, which the Heidelberg Catechism covers in question and answer 113.
If there were any doubt that the Ten Commandments address all of life, the tenth commandment utterly destroys it. After all, coveting deals with our very desires, and if God's law addresses even the intents and motivations that no one but the Lord can see, then nothing is outside the purview of His concern. Coveting is a specific kind of desire, however, and does not refer simply to any desire that we might have. Scripture endorses a desire for God's blessing by repeatedly telling us what the Lord will give us if we follow Him (Deut. 28:1–14; Mark 10:29–30). Moreover, the Bible is clear that Christians can rightly have all sorts of godly desires, including the desire for sexual intimacy in marriage (Song of Solomon), a man's desire to be an elder in the church (1 Tim. 3:1), and many other desires of the heart (Ps. 37:4). To be a Christian is not to be free of desire but to have one's desires rightly ordered and directed.
Covetousness may be defined as the desire to have or seize what is not lawfully ours and the desire to obtain something in a manner that violates God's law. Given these parameters, we can see that covetousness is in some sense a root of all other evils. Theft results when we covet another person's belongings. Adultery is the logical end if we indulge our covetousness for someone to whom we are not married. To worship another god is to manifest a heart that covets a deity that can be manipulated or controlled. We could go on, but the important point to note is that the tenth commandment reminds us that all sin begins in our hearts (Gen. 4:1–16; Matt. 15:19–20a).
We were created to have desires, particularly a desire to know and love our Creator. Covetousness represents desires that have gone awry.
Sometimes it can be hard to differentiate covetousness from legitimate desire. But there are things for which we can be on the lookout. Envy of another's position likely indicates that one is coveting it. Obsession with a particular good to the point that it consumes one's thoughts entirely may reveal a coveting heart. Let us ask the Lord to search our hearts and reveal to us any point at which we are breaking the tenth commandment so that we might repent.