Peter now builds on what he has said about the suffering of Christ and shows the instruction it gives, not only in bearing suffering, but now in putting sin to death. He does this in a way that shows the link between the two.
If you're truly saved, you have a new mentality. You are altogether changed. You have new tastes. You cannot go along with the things you loved before. No longer – and this counts in connection with all your unsaved family and friends and colleagues – [can you] be enthusiastic, and share the same things. You don't watch the same films, the same movies with all their lascivious content, and their appallingly immoral, immodest portrayal of people. In that sense, you've done with sin; you've done with the world. You've finished with it; you’ve fallen out of love with it. You've destroyed all your old music. Previously, you couldn't live without the beat, the drug, the sound drug. It wasn't as bad as heroin or any of the others, but it was a drug just the same, and if you couldn't have it thumped into you, and all those lyrics and all those ideas, your mood dropped and you couldn't survive. There’s a new nature now in you. The old nature is still down there somewhere, and the old ideas still arise from time to time, and seek gratification, but you are no longer under its dominion; it no longer rules you; it's not the biggest thing in your life. You now have a lifelong battle against what the Puritans called residual sin.
The Christian must be constantly remembering his conversion experience and the new man that has been formed in him. This is the most powerful way of defeating sin, far more powerful than the use of raw will power. God has laid the foundation of our sanctification in our conversion experience and this experience must find its outworking throughout life as it changes every part of us until the flesh is finally taken away either in death, or in the twinkling of an eye, at the last trumpet. That principle of life to God has been made the dominant principle in the Christian and it will prevail over the other principle of sin and death. Does this seem a shaky foundation on which to build the very difficult work of sanctification in a believer’s heart? It is the most powerful foundation possible, because it is the work of the Holy Spirit which cannot be overturned.