10 Finally, be strong in the Lord and in the strength of His might. 11 Put on the full armor of God, so that you will be able to stand firm against the schemes of the devil. 12 For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places. Ephesians 6:10-13 (NASB) (Read verse 13 on the site)
I have taught and written much about the battle believers have, and will always have in this life, with their flesh. Most believers are enslaved to their flesh. Why? We were made with an insatiable desire for fulfillment. At birth we inherited our Sin Nature from Adam. We were all born that way. Prior to our salvation we sought fulfillment via it. It primarily utilized our flesh for that purpose. At our salvation through the washing of regeneration, our Sin Nature was cleansed from us.