Exo 20:17 Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's.
Out of all the ten commandments, this last one is probably the least thought of and most shrugged off. While we commonly agree that it is a heinous sin against God and man to commit idolatry, adultery, theft, murder and most of the other ten commandments, the idea of covetousness is not thought of as one of the "worst" sins a man could commit. On the contrary, we live in a culture that heavily promotes the idea of covetousness, one that floods all our senses with the idea that we don't have as much as we deserve. TV, billboards, radio, magazines and everywhere else an advertisement is found, a sense of ingratitude is being cultured, nine times out of ten. I don't mean to say all advertisements are bad, but I do mean to say that the way they are presented often are. This problem persists outside of advertising though, in the business world you are often told things like, "it's do-eat-dog world out there", "look out for number one first" and "trample on whoever necessary to get to the top", such sayings promote the idea that material possessions are more important that people.
While this is the only commandment that deals only with your thought life and not directly with an outward action, does not cheapen it's importance in God's eyes. Take a look at Eph. 5:5 "For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God". This passage is very clear that those who are living in a constant, unrepentant state of covetousness are showing out what's inside, or rather the lack of it, the spirit of God obviously is not dwelling in them and they have no inheritance in the kingdom of Christ or of God. Notice he doesn't say they have a lesser or smaller inheritance, but that they have no inheritance. Covetousness does not make you lose your salvation, but is you are living with it and refusing to repent, you are proving that you never were saved to begin with. If God were to deal with you as a child he would help you defeat that spirit of covetousness (which the bible says is idolatry) whatever correction it took. If he is leaving you in slavery to your sin, than he is dealing with you as a bastard and not as a son. (Heb. 12:8)
Obviously than this sin of covetousness is a very abominable one before God, it is one that is easy to fall into in our culture, yet it must be avoided at all costs. Our souls are at stake here, steer clear of placing things as more important than God or even other people. A man's life consists not in the abundance of the things which he possesses. (Luke 12:15)