Christians seam to have an incredibly hard time seeing Satan unless he's in plain view. We don't live in reality. The Bible tells us the spirit world is more real than, and is the driving force of, the physical world. It also tells us that you can serve either God or Satan, and choosing not to serve God is a choice to serve Satan. Thus the spiritual realm is the driving force that shapes the physical world.
Human beings are the interface by which the physical world is shaped. It's why God works through people. He gave us dominion over the physical universe in the beginning, and not only does He choose to work through us, this dominion granted to us forces Satan and his minions to also work through humans to effect their plans.