For the flesh sets its desire against the Spirit, and the Spirit against the flesh; for these are in opposition to one another, so that you may not do the things that you please.Galatians 5:17(NASB)
As descendents of Adam, we were born in the 'flesh' with an old sin nature which is at enmity with God, but as Christians, we are born again in Christ. As believers, we are born of the Spirit of God with a new Christ-like nature and reconciled to God through the blood of Jesus so that we may not do the things that we please, but those things which honour God.
Before we were saved, we had one single, sinful nature - the 'in-Adam' fleshly nature which is at enmity with God. But at our rebirth, we received a new, second godly nature - the 'born again' spirit of man. We were given a new 'in-Christ' spiritual nature that is eternally united with our Saviour, at peace with God, and clothed in the righteousness of Christ so that we may not do the things that we please.
But from the moment that we became a Christian, the old Adamic fleshly nature started to conflict with our new Christ-like spiritual nature, for the flesh lusts against the spirit, and our new spiritual born-again Christ-nature lusts against the old fleshly Adamic nature within.
God could have chosen to remove the old sinful Adamic nature the moment we trusted Christ as Saviour but He chose to allow the two natures to co-exist within our earthly body so as to remind us of our human frailty and ensure that we maintain a surrendered dependence upon Him - in all things.
Instead of removing our old Adamic 'flesh' God, in His grace, came to indwell our mortal frame - and although our 'fleshly' nature will continue to be in opposition to our new human 'spirit' during our journey through life, we are instructed to yield to the indwelling Holy Spirit of God: "For greater is He that is in us that he that is in the world."
When yielded to the Holy Spirit, our human spirit will grow in grace. It will mature and ripen as we become increasingly conformed into the likeness of Christ, through the work of the indwelling Holy Spirit.
Let us not yield to the flesh, (the old Adamic sin nature) but rather let us yield to the Spirit of Christ so that we may not do the things desired by the flesh - but be increasingly transformed into the person that God wants us to be.
Heavenly Father, I know that I am weak and that my fleshly nature lusts against the new Christ-like nature that I received as a free gift of grace, when I first trusted Christ as Saviour. I pray that I would make the right choices and not yield to the old sin nature but submit to the leading and guiding of the Holy Spirit within. Help me to grow in grace and mature in the faith, to Your praise and glory. In Jesus' name I pray, AMEN.