Those who walk according to the flesh mind the things of the flesh; its mind is death and at enmity with God, not subject to God's law, and those in the flesh cannot please God. 

Those who walk according to the Spirit mind the things of the Spirit; its mind is life and peace; they have the Spirit of God in them. 

If Christ is in them, the body is still subject to death because of sin, but the spirit has eternal life in righteousness in Jesus.