Galatians 4:9, Now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more?
It does not make any sense for a Christian to return to his or her former sins—one’s former way of living as the rest of the sinful world lives, our former masters; for as Christians, we are no longer citizens of this world. “But our citizenship is in heaven, and from it we await a Saviour, the Lord Jesus Christ, who will transform our lowly body to be like His glorious body, by the power that enables Him even to subject all things to Himself” (Philippians 3:20-21). And, “Behold, the kingdom of God is in the midst of you” (Luke 17:21).
As citizens of God’s kingdom, therefore, why return to the world’s behaviour of lawlessness and licentiousness? God’s people are a people set apart (holy). A people of the sinful world is not a people set apart. If you want to live as the world lives, renounce Christ. Brought into God’s kingdom, we live as Christ lives. Though we are in the world, we are not of the world (John 17:11, 14-20). That is, we live in the world for the time being, but we no longer belong to the world but to God’s kingdom.
“Therefore,” Luther says, “the rest of life after Baptism is nothing more than a looking forward to, a waiting, and a longing for the revelation of what is in us [the kingdom of God], so that we may take hold of what has taken hold of us, as St. Paul says, ‘I pursue it to take hold of it, because Christ Jesus has taken hold of me’ (Philippians 3:12)” (Church Postil, sermon for the Second Day of Christmas on Titus 3:4-8, LW 75:237).