There are servants in the castle who may become deceptive secret agents of the rebel leader and use their inside servant role to seduce and capture parts of the castle. These servants are called desires. Do not let sin reign in your mortal body so that you obey its desires . The word is neutral. They may become evil desires or lusts, but not if the rebel sin does not capture them.
Incremental surrender is possible.
That s what the word obey signals in verse 12. Do not let sin reign in your mortal body so that you obey its desires. If sin, the leader of the revolt, takes some desire captive and sends it in behind the castle walls with a deceptive promise of immunity and reward for capitulation, the obedience to that desire would be the surrender of part of the castle. There are weapons in the castle that may be captured and turned around and used by the enemy for his unrighteous purposes. These weapons are the parts of your body your eyes and ears and tongue and hands and feet and sexual organs.