A bit from Kris Lundgaard:
Two enemies, no matter how deep the river of their bitterness runs, can make peace—but only if the hostility between them is destroyed. It is impossible to make peace with hostility itself. So when Paul identifies the flesh with enmity and hatred of God, he cuts off any hope that the flesh will bow to God or befriend him. A treaty between God and the flesh is impossible.
In Romans 5:10 Paul says that we were God’s enemies—we were all of us Captain Ahabs. Christ is the peacemaker in the gospel using his death to put to death the hostility between us and God. Our “old man” (the flesh) was crucified with Christ (Romans 6:6), rendering it powerless to rule over us and enslave us and bear the fruit of eternal death in us. When he appears, he will annihilate the flesh forever. This is the only way to deal with enmity: destroy it.
But ever drop of poison is poison; ever spark of fire is fire; and the last bit of flesh that remains in the believer is still enmity. When God’s grace changes our nature, it doesn’t change the nature of the flesh. It conquers it weakens it, mortally wounds it, so that we are no longer Captain Ahabs by nature; yet his defiant malice smolders in our flesh. By the time Paul wrote Romans, he must have been as Christlike as anyone can expect to be this side of heave, and he surely spent his days putting his flesh to death. Still he cried out for deliverance from this irreconcilable enemy.
What a wretched man I am! Who will deliver me from this body of death? (Romans 7:24)