What we dwell on in our minds will shape the way we live our lives. What you set your mind on shapes your character and behavior.
To set your mind is more than simply thinking about something. It means to focus intently on something, to be preoccupied with it, to let your attention and imagination be totally captured by something. Wherever your mind goes most naturally and freely when there is nothing else to distract it—that is what you really live for.
Christians are not those who live by the sinful nature (or “flesh”) and therefore die (v 13); we will, and must be, those who “put to death the misdeeds of the body.” Paul is saying that we have been made alive (v 10), and we will one day have renewed bodies (v 11); and for now, we put to death the sinful nature, in the power of the Spirit.
This means a ruthless, full-hearted resistance to sinful practice. “Put to death” is a violent, sweeping phrase—it means to declare war on attitudes and behaviors that are wrong. A Christian doesn’t play games with sin—they put it to death.
This also means applying the gospel to our hearts, rather than simply resisting sin in our behavior. We need to remember our obligation to the One who has given us spiritual life now and will give us perfect bodies in the future. Sin can only be cut off at the root if we expose ourselves constantly to the unimaginable love of Christ for us. Sin grows when we think we deserve something from God, or life. Godliness grows when we remember we are debtors to God, throughout life. Putting sin to death is part of what it means to have our “minds set on what the Spirit desires” (v 5).