David declared, "I will fear no evil" (Psalm 23:4). David's hidden man remained unmoved, undisturbed in heart, no matter what Satan threw at him. Why? Because he was fully at rest in God's faithfulness to perform His Word.
David was able to say, "I've had a revelation of my Father's love and patience toward me. Therefore, I will accept no more lies from the devil. I know better than to listen to him anymore, because the Holy Ghost has educated me. Let storms of trouble come, let demons rage, let enemies rise up on all sides. Let sickness and even death stare me in the face. My heart is at rest, because I know all things are in my Father's hands. And He's working everything for my good."
By contrast, hand-wringing Christians have no authority. All they can think is, "Why would God allow this to happen? What am I going to do?" Their lives are full of chaos, fear and murmuring, because they've forfeited all resources. They've neglected to hide God's Word in their hearts, so they aren't able to turn to it in times of crisis.
The only righteousness that frightens Satan away is the righteousness of faith. "The work of righteousness shall be peace; and the effect of righteousness quietness and assurance forever" (Isaiah 32:17). You can't stand against the devil simply because you don't drink or use drugs anymore. You may live by an entire catalog of do's and don'ts, but those aren't the essence of God's righteousness. Righteousness is believing that what God says is true, and committing your life to it. It's that simple.
When Isaiah says, "The effect of righteousness [is] quietness and assurance forever," the Hebrew word used for assurance means confidence. Simply put, faith in God's promise of forgiveness produces an unshakable confidence in us. We may still be sorely tempted, but we know Jesus is at work in us. In short, spiritual authority is this: I walk in full assurance of the reliability of God's Word. I do what it says, submitting to every command. And my faith in His Word to me puts my heart at rest. Satan can no longer linger in my presence. I need merely to say, "The Lord rebuke you, Satan," and he will flee.