For this is the will of God, your sanctification: that you abstain from fornication; that each one of you know how to control your own body in holiness and honor, not with lustful passion, like the Gentiles who do not know God; that no one wrong or exploit a brother or sister in this matter, because the Lord is an avenger in all these things, just as we have already told you beforehand and solemnly warned you. For God did not call us to impurity but in holiness. Therefore whoever rejects this rejects not human authority but God, who also gives his Holy Spirit to you.
Paul, First Letter to Thessalonika, 4:3-8
sanctification: progress in holiness; a greater capacity to live as God's own. It will be impeded to the extent that we dabble in God-ignoring behaviors.
fornication: sexual self-gratification not sanctified in marriage. The great human coping mechanism. People should receive love and respect in the church, not be coped upon by neurotic clergy or predatory lay people.
brothers and sisters: not just a rhetorical flourish of campy preachers, but the way the New Testament defines the people of the church. Church relationships should establish respectful bonds of family affection, not a game reserve for those who work out their emotional stuff sexually. Roman Catholic cover for abusive clergy, Liberal Protestant LGBT activism and Evangelical acceptance of serial marriage all exploit God's people.
Gentiles: from the Hebrew "goiim" or "nations." The church is meant to be distinct from the world around it, a counterculture trying to live for the Kingdom of Heaven. The politicization of churches, right or left; divorce rates on par with the surrounding culture; clergy and people ignorant of Christian distinctives - these and other realities reveal a church that "does not know God."
impurity: Jesus blesses the "pure in heart." In Biblical language, the heart is the seat of the will, the place of our intentions. One need not be prim, prudish and "perfect" to have right intentions - which is why Jesus also blesses the "poor in spirit" and those who "hunger and thirst for righteousness." But the church must hold out the high vision of God's purity, a goal toward which our wills can aspire and, by divine help, progress.