“But” refers us back to the previous verse – the Gentiles walked in callousness, practicing every kind of impurity. We can no longer live this way because in Christ we learned a different walk. I was raised in a Christian home and came to faith at an early age, but there have been many times in my life when I picked the stinky, rotten “old self” off the floor and willingly put it on. Even today when I desire nothing more than holiness I often find myself engaging in some sin (usually of the tongue) and in great need of mind renewal.
In verse 21 Paul seems to insinuate that some of his readers have not truly been taught in Jesus. Even today some in fellowship with the body of Christ may even have vast knowledge but no true life change and salvation.
The old self is separated from God, capable of doing good in the eyes of man but incapable of doing anything but evil in the sight of God. When you are born again you enter into God's kingdom and become a totally different individual. "Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come” (2 Cor 5:17). "I am crucified with Christ: but yet I live; not I, but Christ that lives within in me…” (Gal. 2:20). We don't receive some new set of instructions, we ourselves are made new. There is a holiness without which we will not see the Lord (Heb. 12:14).
Although at the moment of salvation we become completely new creations, the corrupt deceitful desires promising one thing but delivering another do not instantly evaporate. Paul wrote, “So now it is no longer I who do it, but sin that dwells within me. For I know that nothing good dwells in… my flesh. For I have the desire to do what is right, but not the ability to carry it out” (Rom. 7:17-18). This side of heaven we endure the sin which dwells in our humanness. We must strip away the desires of the flesh and replace them with holy garments (1 Peter 2). But instead of changing their clothes some merely tuck in their shirts or thrown on some lovely accessories. They want the hope of heaven without dressing for heaven. We must daily examine ourselves for any trace of flesh within us. As we grow in Christ we will sin less often, but the sins we commit should grieve us more deeply.
You are what you think. We are renewed in the spirit of our minds, not given some list of legalistic rules to follow. We must become a new person, created after the likeness of God in true righteousness and holiness. We seek things above, considering our earthly bodies dead to immorality, impurity, passion, evil desire, and greed (Col. 3:1,5). "We do not lose heart. Though our outer nature is wasting away, our inner nature is being renewed every day” (2 Cor. 4:16-18). So “set your minds on things that are above, not on things that are on earth. For you have died, and your life is hid with Christ in God" (Colossians 3:2-3).
“Our society is hostile to godliness because it is dominated by carnal ambition, pride, selfishness, greed, lust, and a desire for evil. Its opinions are wrong, its aims are selfish, its pleasures are sinful, its influence is destructive, its politics are corrupt, its honors are empty, its smiles are phony, and its love is fickle” (MacArthur).