What does 1 Thessalonians 4:7 mean?

Paul gives another reason to renounce sexual impurity. These are part of his encouragement to the Thessalonian Christians, after a lengthy description of their excellent reputation. The prior verse warned that impurity brings divine judgment.

As stated here, the second reason is that God has called Christians to lead a holy life. God saved us not simply to rescue us from eternal judgment. He intended to make us holy as He is holy (1 Peter 1:15–16). The process through which He makes us holy is called sanctification, and it involves a partnership. God works in us to make us holy (2 Corinthians 3:18; Philippians 2:13); we have a responsibility to cooperate with Him in that work. Philippians 2:12 commands us to work out—not work "for"—our own salvation with fear and trembling. Ephesians 6:10 commands us to be strong in the Lord, and the next verse commands us to put on the whole armor of God. Galatians 5 talks about living by the Spirit. Obviously, the Christian life is not a passive life, but an active one in which God and His people are partners.