“‘And you shall not covet your neighbor’s wife. And you shall not desire your neighbor’s house, his field, or his male servant, or his female servant, his ox, or his donkey, or anything that is your neighbor’s.’” (Deuteronomy 5:21 ESV).
Coveting is a deep-seated longing to possess something that is not yours. Purity of heart is in focus here because to covet your neighbor’s wife is to lust after her – to want her instead of whom you are in a covenant relationship with (see here). Likewise to desire what your neighbor has as better than what you have is the first step towards breaking the 8th commandment of not stealing (here).
God is after the heart (Matthew 5:21-30) and always has been! When we covet something else, we bring it in to our hearts and tell ourselves, “if only I had this then life would be worth living, I would be happy, fulfilled, significant or worthwhile.” This comes back to the first commandment of not having any other gods before the one true God. Coveting leads to idolatry which leads to all sorts of behavioral problems because what is in our heart is what drives our thoughts, motivations, emotions and actions; we are defiled not because of what we put into us, but because of what comes out of us (Mark 7:20-23). Whatever is on the throne of our heart is what we worship.