Paul is referring to the struggle that takes place in the heart of the Christian. He will testify to his own experience of this same struggle in Romans 7:7–25.1 Essentially, when the sinful nature desires to choose evil, the Spirit opposes the sinful nature. When the Spirit influences me to choose good, the sinful nature opposes the Spirit. There is a battle going on in the heart of the believer.2
17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.