Please keep in mind: if you, as a Christian, have an immoral thought, an impure impulse, or show some sign of greediness, this certainly does not disqualify you from entering the Kingdom of Christ!! Even if you should "fall" into a sinful act that could be described with those terms, again, this is not going to keep you from heaven. Just like John will speak in 1 John, sinful behavior that is a way of life is simply what's not true of a child of God. The Christian's way of life is to practice righteousness. When a Christian sins, they feel bad! And that's a great sign that the person is sensitive to doing something they know is not right. A growing believer will want to avoid a sinful lifestyle.
So, here is where Paul wants to emphasize to Christians that this kind of evil behavior should not be part of their lives! If an unbeliever sees a Christian engage in sin, they may conclude that the sin is not all that bad and, thus, be encouraged to continue that behavior. Worse yet, the unbeliever may conclude "why should they become believers when the Christians they know act no differently than they do?!" Believers should desire to be examples of purity and clean living - note 1 Timothy 4:12. They should show the world that Christ leads to a wholesome, refreshing way of life!
An interesting note in this verse is what Paul says about greediness, or "covetousness." It is equivalent to idolatry! Setting your heart's desire on something makes that something the object of your affection and your worship! The only One who should get our worship is Jesus!