Satan has no power over believers—he deceives us into using our own power against ourselves! He uses fear and deception to lead us away from the truth of the Word. We create false beliefs, which in turn create false realities. And since 'as a man thinks within himself, so is he...' those false beliefs create a false identity, which in turn creates a false reality.
For example, 'I could never do [fill in the blank]' is, for a believer, a false belief. The truth says "All things are possible to him who believes."
Jesus repeatedly told people seeking His help "According to your faith, be it done unto you." He was in fact showing us spiritual law, how to operate in life. Faith is based on a person's beliefs, whether good or bad, and creates their own personal reality.