Holiness is a topic that can seem archaic or self-righteous. But in this passage Paul says that it’s essential to life in Christ. He tells the Thessalonian church to be holy, because they are holy. The test-case he uses for their holiness is their attitude to sex, and how much their attitudes about sex have been formed by the sex-obsessed society around them. This is a hard word today, as we all struggle in the same way, but God is a good Father who wants the best for his children: to live in the light of holiness.