Luke 10:17-20. The seventy who Jesus had sent out to preach the Gospel return to him rejoicing and saying even the demons were subject to them. Jesus replies "I saw Satan fall like lightning from heaven." He assures them that he has given them power and authority over all the forces of evil, but he cautions them not to rejoice over that power, but rather to rejoice that their names are written in heaven.

What a heady feeling it must have been for the disciples to realize they now had the power to cast out and subdue demons! When they left, Jesus gave them instructions, and told them that they would be rejected in some places, and that he was sending them out as "lambs in the midst of wolves", but he did not tell them about this power to cast out demons. He let them find that out for themselves. I think that was kind of the point - if they had known about the power they would not have had it. What I mean is, the power over evil only comes from truly and humbly living in Christ, accepting him, his love, his Word, as Truth and preaching those things. The disciples were not to go out looking for a fight, but rather they were to go out preaching in love.

Love and humility are two things that evil has no power to fight, and therefore hates. If Jesus had told his disciples they had power over demons before sending them out they would have approached the entire mission in the wrong way. That is why, even once they realize they have it, and he confirms that he has given it to them, he cautions them not to rejoice in this power.

It would be easy for the disciples, and for us, to make an "idol" of the power the Christ imparts to his followers - that is how power corrupts! It is a good thing that can easily become a primary focus and then, instead of being our power over demons, it becomes the demons' power over us! So we must always, instead, look to heaven as our goal. Having that goal is what sustains the power. If we lose that proper goal, we also lose the power, and open ourselves to corruption.