We all know the basic problem is sin. Second, selfishness takes hold in the life of God's people where there is a failure to recognize the completeness we already have in Christ. As far as God is concerned, we are as Christ is. Yet that is not the popular view. Third, there are the practical issues of morality that Paul raises.
Sexual immorality stands foremost as the greatest danger confronting the fellowship of believers.
Sexuality is the most intimate expression of knowing another, and so to misuse it is to interfere with the knowing of God and of each other. Then there is behaviour which destroys trust. Hurtful violence in words, malice, abusive speech and lying. All destructive of our fellowship, all undermining the very nature of church as a community of believers. The Easter message is the mechanism for the renewal of God's people.