The flesh opposes the work of the Spirit and leads the believer toward sinful behavior he would not otherwise be compelled to do.
The believer does not have two competing natures, the old and the new; but one new nature that is still incarcerated in unredeemed flesh. But the term “flesh” is not equivalent to the physical body, which can be an instrument of holiness.
Our mortal body is the only remaining repository where sin finds the believer vulnerable. The brain and its thinking processes are part of the body and thus tempt our souls with its sinful lusts.
Christians must wait until their glorification before they are finally rid of their unredeemed humanness (Romans 8:23), yet by walking in the Spirit they can please God in this world.
Commentary from the MacArthur study Bible, notes for Galatians 5:17 and 5:24; Romans 6:6 and 6:12.