There are two ways to understand the authority of the believer…
For example, If you look at a rugby match, you see a team of strong players. Well they could represent the devil and his demons. They are strong and powerful players, but they are still under the authority of the referee. Jesus has that authority and Jesus has given us His authority.
Though we are not the referee and in and of ourselves we have no authority, we have been given Jesus’ garments to wear in this “game” called “life on earth”. We have been given the authority to give the devil his red card.
Another example, is that of a policeman who has authority through his uniform just as we, believers, have authority through being in Christ.
The policeman does not have the physical power to stop trucks in any way, but his uniform gives him the authority. We cannot stop the Devil and his demons because of our power but because of the authority we have in Christ.
As truck drivers have to submit to the Policeman’s authority, so the Devil and demons have to submit to our authority in Christ.
Our battle is against evil and setting all who are oppressed of the Devil free!