Luke 10:19

Listen carefully: I have given you authority (that you now possess) to tread on serpents and scorpions, and (the ability to exercise authority) over all the power of the enemy (Satan); and nothing will (in any way) harm you.  

The devil is afraid of the born-again believer!  If the devil can keep the believer ignorant of their authority, he can prevail against the believer.  Knowledge of the word of God is power and authority here on earth.  Jesus gave us the authority to help Him here on this earth.