You wanted to know how it is that a person can change his lifestyle? The answer is simple:
Anyone can change if he is determined to do so. Some change for health reasons (they may stop smoking). Others change in order to please a certain girl—or fellow—as the case may be (they change their language habits, for instance). And, it is true, that there may be certain temporal benefits to the change. Life may be easier. But if the person isn’t a believer, then his change isn’t pleasing to God (Romans 8:8). And in the long run, it isn’t for the better at all. Even a believer (by ignoring the Spirit) may change in the same way—and not please God (cf. 1 Corinthians 3:1-4, where believers were acting like unbelievers). True biblical change is different.
How so? When a Christian changes in ways that do please God it is by
- Doing so because he wants to obey and honor Him. He seeks to make a certain change because it is God’s will for him to do so—as he finds it in the Bible; by
- Doing so by replacing sinful ways with righteous ones (or as the apostle Paul put it: by putting off the former and putting on the latter; Colossians: 3:9); by
- Doing these things through seeking the help of the Spirit a) enabling him to understand God’s will in the Word; b) empowering him to make the changes involved (cf. Philippians 2:13).
Thus, in essence, it is change that one makes in consort with God Himself. Thus both the motive and the method are of God as well as of the believer.
 To be in the flesh is to be without the Spirit of God.