When we become believers we have a new relationship with God through the Holy Spirit. The Spirit comes into our lives and remodels our soul. What are some things which can be remodeled by the Holy Spirit? Well there are many things. Our emotions, our outlook on life, our morality, our joy, but the area Paul emphasizes in this passage, in other passages of Romans and in other letters he wrote was the renewing of our mind. The mind, and the potential of the mind, is a very big deal in Paul’s theology.
Those who live according to the sinful nature have their minds set on what that nature desires; but those who live in accordance with the Spirit have their minds set on what the Spirit desires.