In making an arrest, a policeman may be physically and even mentally weaker than the suspect; yet he can make the arrest. Why? Because he has authority (badge) and power (gun and handcuffs). His power and authority are not self-derived, but are delegated to him from a higher legal source.
In constraining forces of evil, we Christians have both power and authority. Our power comes not from ourselves but from a higher source: the Holy Spirit within, and our authority comes from our personal relationship with Jesus. But Satan too has both power and authority. And even his demons have these tools.
In our daily spiritual warfare we can do more harm than good by trying to use our own power and self-assigned authority (see Mk. 9:14-18),without consciously being aware that in confronting the Enemy we are strong “IN” Jesus (Eph. 6:10-18), not just “FROM” Jesus. Even St. Michael the Archangel let God rebuke Satan and wouldn’t do it himself (Jude 9). Jesus reminds us that we can cast out demons only in his name.
We can be strong only in Jesus’ strength and power; our strength comes only from a conscious and spiritually intimate personal relationship and dependence on him. This is because Jesus has defeated Satan.