Luke 9:1, 2-"He [Jesus] called the twelve together, and gave them power and authority over all the demons, and to heal diseases. And He sent them out to proclaim the kingdom of God, and to perform healing."
Notice how Jesus equipped His disciples for ministry. He knew that when they began preaching the kingdom of God and healing the sick, demonic powers would bring opposition. So He specifically gave them power and authority over demons. Later Jesus sent out 70 of His followers on a similar mission, and they "returned with joy, saying 'Lord, even the demons are subject to us in Your name'"(Luke 10:17). These missionaries were spiritually in tune enough to know that demons existed and that they were a force to be reckoned with in their ministry. Jesus' followers had been eyewitnesses as the evil spirits opposed the Master, and they probably anticipated the same treatment. Perhaps they even started out on their mission with pangs of fear and doubt about encountering demonic resistance. But they came back astonished at the victory they experienced over evil spirits. But Jesus quickly brought the issue of spiritual conflicts into perspective: "Do not rejoice in this, that the spirits are subject to you, but rejoice that your names are recorded in heaven" (Luke 10:20). Jesus sent out the 70 to preach the gospel and to heal, but all they could talk about when they came back was how they sent the demons running. "Don't be centered on demonic spirits," Jesus replied. "Be kingdom-centered, be ministry-centered, be God-centered." That's a good warning. As you learn to exercise authority over the kingdom of darkness in your life and in the lives of others, you may be tempted to see only the spiritual battle, looking for demons behind every door. But it's truth which sets you free, not the knowledge of error. You are not called to dispel the darkness; you are called to turn on the light. You would have no authority at all if it weren't for your identity as a child of God and your position in Christ.