James 4:7 - "Submit yourselves, then, to God. Resist the devil, and he will flee from you." (NIV)
Our relational submission to God is what the devil notices. On our own, we have no power. We are no threat to the enemy. But, when we are submitted to God, his power comes through us. We are the conduit of his authority. It's not our own personal authority here.
In the same way, Jesus told us, "Remain in me, and I will remain in you. No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me. (John 15:4 NIV). Our connection to the power source is important.
Notice that Jesus tells us to resist the enemy. He doesn't tell us to ask Jesus to resist the enemy. As submitted children of the King, we speak out, "I resist you, devil, in the name of Jesus (recognizing his authority). God promises that when we take up our authority, the devil will flee. Not maybe he will run away. He WILL flee. That's God's promise.