We're all familiar with Ephesians 6:12
For our struggle is not against enemies of blood and flesh, but against the rulers, against the authorities, against the cosmic powers of this present darkness, against the spiritual forces of evil in the heavenly places.A lot of spiritual, emotional, and psychological harm has been done in the name of spiritual warfare against demons. Because of this - and because, frankly, our scientific age has a hard time believing in imps trying to do us dirty - many Christians today don't know what to think about demons.
There's the liberal position that demythologizes demon-talk, stripping out all the supernatural and leaving only the existential. But this is also unsatisfying because we still believe the language of evil and the demonic points to something real. Take everyone's favorite example: Hitler. We can try to explain him in scientific and political and psychological terms - and yet there's still something about him that goes beyond all those explanations. There is a residual*, which we call demonic.
Yoder and Stringfellow and Wink point to a way of understanding principalities and powers that takes into account the mixed ways the Bible talks about them. At times the writers seem to be referring to actual beings; at other times they seem to be referring to institutions.**
In short, the powers are structures created by God and necessary to human flourishing. Think governments or religious institutions or ideologies. Though they were intended to serve humanity they tried to set themselves in the place of God. They are in rebellion against God and they seek to enslave humanity.
So in this understanding the powers are not invisible demons but aspects of the world's system. Spiritual warfare is not praying that angels fight demons up in the sky. Spiritual warfare is confronting the institutions that set themselves up as idols and calling them to their proper place.
There is much, much more to say on this but I'm running short on time this morning. There is an excellent blog series by Richard Beck, beginning here, which goes into more detail. And, of course, I'd recommend the books mentioned above.
* - The idea of the residual is one I learned from Richard Beck.
** - See Beck's fourth post for the biblical texts.