God specialises in uniting the world; the flesh and the devil divide. Many who had been converted and joined the church in Corinth had previously indulged in the immoral practices associated with pagan temple worship. When they came to Christ some did not see why their habits should change and so Paul explained that intimacy of that kind was designed by God to strengthen the unity of a man and woman (Genesis 1:24).
 
Although Greek philosophy taught that physical experiences were unrelated to a person's spirit, Paul said that was untrue. Every interpersonal encounter changes both people, especially when those encounters are physical. For the Christian, whose spirit is united to Christ, casual physical intimacy also offends the Lord greatly. The very idea should be unthinkable (Matthew 5:28).
 
Immorality has its own allure and it cannot be reasoned with, so Paul instructs the church to run away from it as though their lives depended on it. The sin is not just against one's spouse (or future spouse) and against God, it is also against their own body – in a way that other sins do not have such a personal impact. Such sins also affect the church which was why Paul instructed them to expel an unrepentant immoral person (1 Corinthians 5:13).
 
This teaching is as necessary now as in the 1stC. Our previously Christianised world has not run away from immorality bringing divisions between spouses, frustrating worship and damaging whole generations of people. They may have thought they were being 'modern' but this is as old fashioned as sinful humanity. Paul's command to 'flee immorality' was Joseph's rule of life (Genesis 39:12) and should come with fresh urgency to a church called to be holy in a world riddled with temptation (Proverbs 7:24-26).