Unseen Enemy

Okay, so here's the question I've been mulling over the past...I don't know...lifetime. I've been raised to believe that there is an unseen enemy. From different events in my life, I have been inclined to believe that is the case, but I've been listening to a podcast by Mark Driscoll and he mentioned something I hadn't ever considered. In his four part podcast "Spiritual Warfare" he stated that there are Christians who do not believe in unseen enemies. Is that true and if so, what are their ideas on the "dark side of the force"? I'm just curious. Any thoughts?