5 For be sure of this: that no person practicing sexual vice or impurity in thought or in life, or one who is covetous [who has lustful desire for the property of others and is greedy for gain]–for he [in effect] is an idolater–has any inheritance in the kingdom of Christ and of God.6 Let no one delude and deceive you with empty excuses and groundless arguments [for these sins], for through these things the wrath of God comes upon the sons of rebellion and disobedience.7 So do not associate or be sharers with them.8 For once you were darkness, but now you are light in the Lord; walk as children of Light [lead the lives of those native-born to the Light].
V5 Paul here gives a quite serious statement about those who partake in certain sins: the sin of sexual impurity, and the person who is covetous. He mentions sexual vice, or impurity in thought or life. This covers all of lusting. Summing it down, Paul is saying that if you practice lust, you will not have any inheritance in the kingdom of Christ and God. This does not say that if you at times struggle with it, but if you practice it. Where does the struggling become the practicing? That is a rather easy question to answer. It becomes a practice when you cannot have power over it in your life. The lust tells you what to do, and you obey, rather than your moral fortitude.
V6 We are to not allow those who are out to deceive us gain any ground in our lives. Those give empty excuses and groundless arguments. Paul tells us to not associate with them.
V8 We were once in darkness, but now we are light in the Lord, so we should walk as children of the Light.