3But sexual immorality and all impurity or covetousness must not even be named among you, as is proper among saints. 4Let there be no filthiness nor foolish talk nor crude joking,which are out of place, but instead let there be thanksgiving. 5For you may be sure of this, that everyone who is sexually immoral or impure, or who is covetous (that is, an idolater), has no inheritance in the kingdom of Christ and God.
Why is Paul saying this? He has just stated that we must walk in imitation of God, so why the specifics?
I think it has something to do with the fact that new converts (and old too) come from all walks of life. They need to be shown what it means to walk like Jesus walked. It might be easy for some to justify such behavior, either in ignorance, or in stubbornness of heart. Either way, Paul is making it clear that these behaviors are out of bounds for the lover of God.
Those last words are strong! What about grace? Grace is all over Paul's writings. But so is the idea that walking in Christ is not merely "accepting Him into your heart" one time, nor is it the idea of get baptized and walk however you wish. Those kinds of responses are idolotrous, and are not for those of God's kingdom.
Ouch! (But if taken to heart, that is a very good ouch.)