I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. -Galatians 2:20
Concerning this verse, John Stott writes:
In Christ “old things are passed away” and “all things are become new” (2 Cor. 5:17, AV). This is because the death and resurrection of Christ are not only historical events (He “gave himself” and now “lives”), but events in which through faith-union with Him His people have come to share (“I have been crucified with Christ” and now “I live”).
Once we have been united to Christ in His death, our old life is finished; it is ridiculous to suggest that we could ever go back to it. Besides, we have risen to a new life.
In one sense, we live this new life through faith in Christ. In another sense, it is not we who live it at all, but Christ who lives it in us. And, living in us, He gives us new desires for holiness, for God, for heaven. It is not that we cannot sin again; we can. But we do not want to.
The whole tenor of our life has changed. Everything is different now, because we ourselves are different. See how daringly personal Paul makes it: Christ “gave himself for me.” “Christ…lives in me.”
No Christian who has grasped these truths could ever seriously contemplate reverting to the old life.